Beneath an important city of your campaign setting, there are no doubt countless catacombs, condemned sewers, old thief hideaways, ancient vaults, and buried-over ancient ruins from past civilizations. But at some point, somewhat uncovers a hole in the city that leads straight down into tunnels that look like smooth stone- but are actually made of a cold metal. What lies within?
Mapping the Tunnels
Before you can begin exploring the tunnels, the DM needs to map them out. Maps don't have to be exact here, no grid squares or exact measurements, but we'll want a general layout, rooms, and connecting pipes.
In order to build this; take the city map of the town or city your players find the tunnels within. Forward thinking GMs can put these beneath the sewer map they already have, and really forward-thinking GMs can base these tunnels on the ancient ruins the city was built over, instead of the city itself. Since I doubt most people have that level of prep- just cut out one or two districts of the modern city to complete the effect of these tunnels being built under the "ancient" stuff.
Take the city map and use a line tool in any image editor to make lines across the main streets of the city. These will be the main tunnels. Whenever a road ends, hook the tunnel to the nearest end point of another tunnel. Don't leave any open ends on the "main" tunnels. I'm going to use this medieval map of Ferrara just as an example, since it's not too big or accurate. We don't have to use every street- we'll say that those were all future additions to give it the "ancient underground" feel.
Now let's close off the ends. If we have an odd number of ends close to each other, or an "end" that is stuck in a block of other tunnels, we'll make that a "main entrance". These are open mouths of the tunnel that lead up to the surface, in the form of manholes, large gaping holes opened from a sinkhole, the door to the tunnels in the basement of an ancient noble mansion, etc. We'll mark these with a funnel.
Finally, we need to stock this map. The lines are the tunnels, which can be walked down like a street. They are sloped at the base and light amounts of metallic water flows through them, but can be traversed by anyone on foot. The tunnels are large enough for three men to walk abreast with ease. They just keep going on, curving and meeting at the crossroads. The area between the tunnels is just filled with earth and rock- but something may be there in each cell formed by the tunnels. Each block has a 1 in 6 chance of something being there. Mark and number these spaces in preparation for keying the map.
Roll for each keyed location.
Keying the Tunnels - d6 Table
 Shortcut. Single straight tunnel bisects this space; useful as a shortcut.
 Tunnel maze. Select the two longest sides of this space-between-tunnels. Roll 1d6 for each side. There are that many entrances to smaller tunnels within the cell. For uneven numbers, the small tunnels must split or merge between the ports.
 Rooms. 2d4 large, rounded-rectangular rooms made of the Blue-Green metal which connect to each other as well as the tunnels outside. 1d3 entrances to the tunnels from the room.
 Terminus. Another tunnel juts off into this space, dead-ending at a certain point. Has a 1 in 3 chance of having a secret entrance to the surface, which must be opened from this side.
 Great void. This space is hollowed out. Massive underground chamber. Within are some fungal blooms and animals or monsters befitting the great underdark. One entrance from tunnels.
 Street. Select one of the lines making up this cell at random. Along that tunnel are small entrances to various "buildings" carved into the stonemetal. Most are ancient and half or fully collapsed. These buildings look like they were carved after the tunnels were first made.
Blue-Green Brass Tunnel Encounters - Roll d8
 You hear a loud, resounding dripping sound. It gets louder each exploration turn, as though its getting closer. If anyone in the party gives a genuine prayer, the noise goes away. Otherwise, roll a 1d2+4 on the Wandering Monster table after 3 turns of the dripping noise following you; you finally encounter what has been following you.
 You find a corpse of a few individuals. They are dressed in clothes that went out of style two or ten generations ago, and their bodies are mostly preserved. They are remnants of an ancient family that tried to escape a catastrophe in the city by hiding in these tunnels; which were already here when this happened. Something like Renaissance explorers finding the remains of some people who tried to escape the Mt. Vesuvius eruption in Pompeii.
 This room or section of tunnel is lit with glowing blue-green witch flames. They float in the air unless molested, quickly falling apart and going dark. If you capture one in a glass jar, you can use it as an unlimited fuel lantern of a lower light quality. You can collect all of these witchflames and sell them later; but this takes a considerable amount of time.
 You come across a small, sweltering "garden" in a section of tunnel or side room. It's very humid and smells awful do to the manure and vegetable matter dragged into this area. There are a few rats picking among the wilted plants and fallen, rotted fruits.
 An unusual divot in the walls clues you in to something there. Each turn you investigate it, there is a 1 in 6 chance that you find a secret button that leads down to an even deeper, darker level of tunnels.
 You find an 80+ year old, raving, blind, insane person lost in the tunnels. Their skin has turned blue from drinking the metallic water in the tunnels and their face is caked with rat gore from the only thing they can find to eat. If you try to talk to them or disturb them in any way, they attack (as 1 HD monster). Otherwise, they just sit there and fester away. If you somehow get them out of the tunnels alive they go back to normal the moment they feel the sun on their skin.
 There is a pothole in the center of the floor of this tunnel here; sticking a 10ft pole in reveals it is actually very deep, leading to an underground aquifer. Requires some work with picks or magic to widen it for exploration.
 You find a treasure chest, painted and corroded to exactly match the walls of this tunnel. It's so well disguised somebody trips on it and that's how you find it. The treasure chest contains one treasure; roll on the Tunnel Treasure Table below.
Blue-Green Tunnel Wandering Monster Table - Roll 1d6
 Long Limbed Zater (1+1 HD, +2 AC, 1d3 Reaching Claws, Jittery)
The Long Limbed Zaters are a pest species. Shorter then man, but with comparatively longer limbers, they move through jittering, cringing movements and jump at any noise or resistance to their probing clawed fingers. Their long arms are weak but have reach similar to a spear. These creatures are constantly on edge and are never surprised- in addition, they succeed on all saving throws if they are done from surprise; such as from a silent sneak-attack spell.
 Smog Ghost (2 HD, Ethereal, Poison Gas)
Smoky green clouds in a vaguely humanoid shape. It is a ghost, and can be Turned or struck with magic weapons and spells. Since they're made of smoke, they can also be blown away by a magical spirit fan or a gust of wind from another spell. The only way for the spirit to "attack" is to grab people and stay close to them so they breathe it's poisonous gas. If a ghost is on top of you at the end of a round, take 2d6 poison damage; save for half.
 Spike Gremlin (1 HD, +4 AC, +1 To-Hit, 1d6+1 Magic Spikes, Camouflage)
Numbers: Just one
This tiny creature is a malevolent, violent trickster. It's color is exactly the same as the walls and floors of these tunnels, letting it sneak around and escape combat if it can slip out of your sight. The gremlin uses Blue-Green spikes it carries on his back and jabs them into things with both hands. These magic spikes stick into whatever pushed into, requiring an exploration turn to remove- prefers to aim for feet.
 Black Mole Swarm (3 HD, +2 To-Hit, +2 AC, Xd4 Bites, swarm, scared of light)
Numbers: One (Swarm of hundreds)
Tiny, pitch-black moles swarm together in a huge group crawling over walls and bodies to find flesh and bones to eat. While there are technically hundreds of creatures in a "swarm" of black moles, they are counted as one creature. Because they are a swarm, each weapon attack can only do a maximum of 1 damage to a swarm, as you are only killing one member of the swarm. Area of Effect attacks or spells can do full damage to the swarm, since it can kill many moles at once.
These moles are also afraid of light. Bright flashes from a spell or flashpowder bomb can cause an instant morale check. Torch and lantern lights on their own aren't enough; it needs to be a sudden burst of light, as though a hole to the surface with the dreadful sun just appeared.
 Wet Mouthlets (4 HD, +3 To-Hit, 2d10 bite, weak legs)
Mouthlets look like a large fleshy ball on two bowlegged, spindly old man legs. There is a tiny pinhole in the center of their mass that opens to reveal their mouth, which is almost as large as the entire fleshy ball of their upper body. They are essentially just a mouth with a tiny stub tail. They have no eyes or ears, and seem to navigate purely by taste. They are always ravenously hungry and take big bites out of anything warm and big enough to excite them- bloody chunks of meat will allow safe passage or the ability to poison them.
These mouthlets match the color of the Blue-Green tunnels; making them hard to see in darkness. If you're quiet, you might hear them, as they are constantly dripping with mucus and dampness from the tunnels down here. While the beasts are very aggressive and dangerous, their legs are very weak and barely hold up their weight- a solid kick or blow with a weapon to a mouthlet's leg will always break it on an attack roll of 16 or better; leaving it helpless. If you run away after this; any of the fallen will be quickly devoured by the other members of their pack.
 Climbing Scuttsman (2+2 HD, +1 To-Hit, +5 AC, two magic knife attacks at 1d4+1, climbing, bombs, roll up)
Numbers: 2d4+2 in a band, 2d20+4 at nest
The Scuttsman are a noncommunicable race of intelligent beings that live within the tunnels. They appear a bit like pill-bug or "rolly polly" people, but more slender. Their color is a dull blue that is not perfect at replicating the tunnels, but close. They also wear basic clothing; typically just a loincloth and a shawl. Despite their apparent organization and level of intelligence, they don't seem interested or capable of speaking any languages and (usually) cannot be bargained with. Whenever you encounter a group of scuttsmen, roll a reaction check.
The scuttsmen travel mostly by climbing on the walls or ceilings of the tunnels. At least one scuttsman will be dripping something from a perfume lantern as they travel; which is essentially a scent marker so they don't get lost. If you have dogs or a sharp nose, you can follow this scent back to their nest. They don't wear armor but do have weapons; crafted magical knives made of a dull bronze metal that are magically enchanted; each one carrying a pair. These knives appear customized, with different scratch patterns or perfumes adorning them as per individual scuttsman.
Additionally, each scuttsman group will have one leader who carries small orbs made of the same dull bronze. These are bombs which break apart when thrown- each leader carries 1d6 bombs and knows what each one does for tactical usage. Roll on the table below to generate the bombs. If you encounter a full scuttsman party of 10 members, or intrude on a nest, two of the scuttsmen will be leaders carrying bombs instead of just one.
Scuttsman Bomb Table - Roll 1d8
 Ammonia. It's not really a bomb, just a container of what is probably their waste. If one of these hits you directly (attack roll) then save or get some in your eyes, taking a round to clean it out. These "bombs" are also thrown on acidic slimes or on their own allies who are coated in acid- the smelly fluid neutralizing the corrosive agents. Against slimes; deals 1d8 damage.
 Flash Bomb. Contains silvery powder which explodes into bright flash when struck or thrown hard. This scuttsman carries the bomb to counter the swarms of black moles who live in the tunnels. Blinds everyone nearby for 1d2 rounds unless you don't have eyes or are naturally immune to the flash (like the scuttsmen).
 Centipede. The inside of this bombshell contains one long, curled up sleepy little green centipede. When thrown it will wake up from the shell breaking and be very angry; biting the nearest individual. The centipede's bite deals one damage and you are paralyzed for 2d6 rounds from its venom; save for half. If you manage to deflect the bomb shell in motion with a spell or prepared parry with your weapon then the centipede will bite whoever is closest when it breaks or just scurry away to the nearest dark place.
 Freezing Bomb. This bomb contains an icy chill wind and freezing energy- anyone struck by it is frozen in place for a round and takes 1d8 cold damage. If it hits the ground, creates an ice patch that causes people to slip in fall if running at max speed. It can also be thrown against a door or two of the bombs can be used at once to create an ice barricade that is very difficult to break through. The ice takes up to three turns to melt given the cool air of the tunnels.
 Disquiet Bomb. This bomb contains a strangely shaped noisemaker which clangs a loud, discordant note when thrown against the ground hard, and in addition to this releases blue mage-poison. The discordant note forces a saving throw against any magic spells cast this round; if you fail the save, you lose the spell and it has no effect. The mage-poison is a glowing blue cloud that dissipates quickly- the bomb either has to hit you directly or has a random 1 in 6 chance to travel towards you if nearby. If you breathe the smoke, you lose the lowest level spell and spell slot you have prepared and feel strangely sick for a few days. Characters who cannot cast spells feel nothing from breathing this smoke. These two effects in combination are used to protect the Scuttsman from the occasional magic user or ancient lich, who they are terrified of and have no good defenses against.
 Blade Roller. What appears to be a bomb actually hits the ground and then rolls, spinning blades on the sides with a mechanical spring inside. This device is perfect in these tunnels, since the walls are sloped, letting it rip around the floor of a tunnel while the scuttsmen can just hop up to avoid it. The blade roller spins around the floor and everybody standing on the floor has a 1 in 6 chance to be in its way as it bounces around; if it passes you it deals 1d4 damage to your ankles and you must save or fall prone. If it hits you while prone you'll take 1d6 damage instead as it tears you up. The roller can be stopped by just smashing it with something hard, but it moves fast, so it has an AC of 16.
 Flame Bombs. Filled with combustible material and explosive. Deals 3d6 damage on a direct hit, 2d6 damage on anyone adjacent, and 1d6 damage on anyone adjacent to them. Save for half.
 Corrosive Bomb. Filled with acid and metal-eroding fluid. Anyone hit by it loses their armor, with a save to avoid your weapon and shield being rusted away with it. If the bomb misses you closely, some of the fluid will splash on you and reduce your armor by -1d3 AC points until repaired. The corrosive also damages flesh; a direct hit means you take 1d6 damage per round until cleaned off with water or until you take a total of 20 damage, whichever comes first. Magic armor and weapons are resistant or immune.
Tunnel Treasure Table - Roll 1d10
 Crossways Star
This emblem is made of two triangular stars, long and thin, crossed over each other. One is silver, and the other is gold. The crossways star is a special magical emblem that can be placed in stone recesses at certain places in the catacombs to open up a new path between two tunnels, or open a hidden entrance to the tunnels from a back alley or basement in the city far above.
For game mechanics; assume any given cell between multiple tunnels paths (see mapping session) has a 1 in 6 chance to have a secret tunnel opened by this emblem; this only applies to cells that do not already have a feature filling them. Secondly; any given back alley, bathhouse boiler, or underground cellar of a nobleman has a 1 in 20 chance to have a crossways star recess, leading down to the tunnels.
Even without it's special properties; the Star is valuable and could be sold for 10,000c
 Tunnel Shaper
This is a dark brown magic wand with a silver wire wrapped around it; giving it an appearance of a drillhead. If you point the wand at a wall while holding it and mentally willing to happen, the wall will begin to widen and shape into a tunnel wide enough for 1 person to walk abreast. You can force the tunnel to shape into a turn, but cannot widen a created tunnel further.
Within the Tunnels of Blue-Green Brass, the material of the walls is softer and better to tunnel with this wand. You can tunnel up to 300 ft of tunnel length per day. In any other dungeon or cave, the hardness of the walls influences how fast and deep you can tunnel through the walls- with an average of 200 ft per day for other places.
 Mage-Bane Gemstone
This bright blue gemstone is cut in a flat diamond shape, and glows when gripped by a living person. It converts a person's energy into blue mage-poison gas. The bright blue gas floats on the air and quickly disperses unless in an enclosed space. It can also be blown away by wind. The person holding the gem has no special defense against the gas; and they'll probably be the first person to breathe it in in any enclosed space since it billows out of the gem when concentrating.
Every round you use the gem, everyone adjacent to you must save to take a step away and avoid breathing the smoke. If they want to stay adjacent, they automatically breathe the smoke. Breathing the smoke makes you the lowest level spell you have prepared. Also, the user of the gemstone gets a -1 to their To-Hit, AC, and saving throws from becoming exhausted, requiring a turn of rest to recover.
 Bomb Crate
This secret stash contains a crate of bombs, used by the Scuttsmen. It is a large box (probably stolen or reclaimed from the surface world) with bone slats inside to create cells containing bombs. There are 40 random bombs. They are not labelled.
 Crystal Dog Skull
Appears like a dog's skull made of white and blue crystals. The bluer crystals form the top of the skull trailing to the tip of the nose, giving it an impressive look from dead on. If you point the skull at a dog, wolf, hellhound, or any other canine, it will freeze up and just stare at the skull until it is taken away or something blocks its vision.
The crystal dog skull is also very valuable and could be sold for 25,000c
 Golden Nose Plugs
Small golden plugs that are well fitted to a humanoid's nostrils. There are only two plugs, so if you have some freaky mutation it won't work 100% of the way. Simply having these plugs in your nose makes you immune to all dangerous miasmas, toxic gases, horrible smells, or other gas based attacks or spells. You can still breathe through your mouth while wearing these and be immune to the magic.
 Staff of the Hills
Gnarled walking stick with a red gourd. The bottom of the staff is dirty and impacted from being used to strike the ground for who knows how long. If the gored is removed from the staff, it will lose its magical powers until tied back onto it. The gourd continually refills with a medicinal water that grants stamina, counts as a single ration, and heals 1 hit point per day, perfect for traveling long distances. The staff itself is not magical but is the only one that will draw power through the earth and refill the gourd with its medicine. The staff can also be used as a standard d4 weapon.
 Revolting Rebec
Small wooden instrument with three strings. The strings are dark red, giving the impression of being plucked from a body, and the wooden is rotting, yet still strong and whole enough to make a note. The instrument can be played with anyone with instrumental skill to play a song; random plucking or a fast bow does not active its effect.
The Revolting Rebec causes a morale check in anyone who cannot see it. Anything with 5 HD or higher is immune.
 Silver Scraping Scarabs
These magical silver body-scrapers are medicinal or cleaning tools in the shape of scarabs with a sharp lip, and can be used to scrap away scabs, body hair, or scum off of the body's surface. Any non-magical healing action done with these scarabas, such as bandaging wounds or performing minor surgeries, will be improved by +1 or +15% chance of success without infection, etc.
Additionally, the scarabs are valuable. They can both be sold as a set worth 5,000c
 Magic Bug Net
Generic looking hoop-shaped bug net with a bronze-orange metal making up the handle and hoop. The mesh net itself is made of a very fine, transparent material that makes it almost impossible to see. Anything placed inside this net cannot come back out unless the net is turned upside down and it is shaken out. This means a hand or foot of a large creature will also be trapped, a small creature would be totally trapped within, even a spell or magical blast can be trapped; though it will fizzle out or damage whatever else is inside the net as per normal. The mesh netting is essentially indestructible, but the handle and hoop of the net isn't; count it as a corrosion-immune +1 magic material.