SubMundo

Intro
The world of Mystera is made up of many floors, with the surface map of Galebrook being floor 1. Directly below is floor 2, the Underground, and below that is floor 3, the Deep Cave. These floors can be reached by building stairways.

On the Deep Cave floor you can find an existing hole (or dig one of your own with a shovel) to enter the Underworld, which spans floors 4 to infinity. Each floor is generated when first reached by a player, and then resets every seven days.

Each Underworld floor is a labyrinth of rooms and tunnels with destructable walls. You cannot build stairways in the Underworld, however you can use a shovel for a chance to dig a hole to the next floor down to speed your descent.

The types of monsters found are more dangerous as you descend deeper into the Underworld; however, gem rocks, altars, skeletons of dead players, statues of mercy, chests and more can be found in greater and greater concentrations.

Exiting the Underworld
While each Underworld level is linked by generated stairways, there are no stairways exiting the Underworld itself. Nor can any be built; in order to leave the Underworld you must: use an escape rope (while on the topmost Underworld floor in order to reach the Deep Cave floor and leave from there), use an Escape Lantern, or find a Statue of Mercy.

Escape ropes are crafted, and allow you to travel up one floor each before being consumed. Statues of Mercy are interactive objects that can spawn on all Underworld floors and which teleport you to a random location on the surface. Escape Lanterns, dropped by Flame Demons and Dust Devils, do the same unless you are bound to a Recall Tile.
 * Escape Rope: 20 tinder, 5 silver.
 * Recall Tile: 1 dragon scale, 1 escape lantern, 10 gold

Recall Tiles can be built on the surface and you can walk on them to bind yourself to one of them.

Underworld Recall Tiles
There are also (different) recall tiles in the Underworld, which can randomly spawn on all Underworld floors like Statues of Mercy and other interactive Underworld objects can. If you walk on one you become bound to it and can teleport to a random location on that floor using a Depth Recall potion, significantly shortening return trips to the Underworld.
 * Depth Recall: 1 dragon scale, 1 blue dye, 1 gold

This does not overwrite any Recall Tile on the surface you may be bound to, however; or visa versa.

Objects

 * Chest: can spawn loot of quality that scales with your questing level and the level of the dungeon.
 * Dark Chest: spawns a small amount of gold, a water, a chaos, and a skeleton lord
 * Crystal Rock: a rock which, when mined, can yield gems which are used in smithing
 * Skeleton: If a player dies in any way, their remains can be found on the same underworld level as their player level was on death
 * Table: Created for the monsters in the underworld to dine on. Destroy it for wood and more spawns. No, a player probably did not place it and you're not stepping on anyone's toes by destroying it.
 * Altar: something about multiplying items, making you hungry or full, spawning monsters or rarely a dog. wew dog.
 * Fountain: Fully restores your HP.
 * Statue of mercy: Takes you back to overworld.
 * Underworld recall tile: Reminds you of your position in UW. By using a Depth Recall potion you are able to start at last stepped recall tile.

Traps
Traps cannot be seen until stepped on, "consuming" them.
 * Mud: Slows you for -800 movement speed. Move around to get rid of the slow.
 * Pitfall: A hole opens up to the next floor; you'll probably fall in.
 * Spike Trap: Deals damage when stepped on.

Items
Sometimes found in chests.
 * Blunt Dagger: A comparatively weak weapon.
 * Dark Hero Platemail: Heavy armor.
 * Dark Memento: Assassin/Destruction accessory.
 * Leather Armor: Medium armor.

Object Spawn mechanics
Each floor can house a certain number of spawned objects (rocks, altars, fountains, etc.), and once that number is reached, little more will spawn. However, if you break a table, mine a rock, use up a fountain, or otherwise remove something which was spawned, another item can spawn. This does not account for what was used up, meaning that a destroyed table can yield an altar and a used up fountain can yield a regular rock elsewhere in that underworld level.

Monster Spawns
Monsters of any level can spawn at any point in the underworld but will typically stick to the floors outlined in the table. This is a result of abnormal spawns from altars and the "rare spawn mechanic" which will occasionally place a monster from many floors deeper in the underworld in a more shallow floor.