The Caverns of Chaos is the most important dungeon in ADOM, and one of two dungeons that are absolutely essential to visit in order to win the game, the other being the Tower of Eternal Flames. It is the home of the Chaos Gate, which is the source of all the corruption in Ancardia.
The Cavern of Chaos is the second biggest dungeon in the game (behind the aptly named Infinite Dungeon), at fifty levels plus side branches, which will be detailed further below.
As befits a dungeon of its size, the Caverns of Chaos have many special levels with guaranteed features. It is also the only dungeon in the game to have "sub-dungeons", that is, dungeons leading off from the main CoC to other special locations. These special levels and guaranteed features will be listed in order.
The Shortcut Level
The shortcut level is not guaranteed to be generated. On D:1 or D:2, you may discover two down staircases, one of which will lead to D:2 (or D:3), and the other to S:2 (or S:3). If a shortcut level is generated, it will bypass D:2 (or D:3) and give you a second way to get to D:3 (or D:4).
Reportedly, it is impossible to get a vault generated on the shortcut level, while the level avoided by the shortcut will have a heightened chance to contain a vault, however this has not been empirically verified.
See main article: Arena.
The next special level in the Cavern is the Arena, which takes up half of the level on D:3 to D:5. Aside from the arena itself, which offers opportunities for the player to fight random monsters in return for sums of gold or even become its champion, the arena level also features the only guaranteed Ratling traders in the game, who will offer cheap, abundant and lightweight sources of food as fried bats, cooked lizard, as well as other stuff, which is uniformly not worth it.
The Big Room
See main article: Big Room.
The Big Room can be found around D:6 or D:7. As the name hints, it is just a giant square room. It is guaranteed to have Herbs growing on the level, as well as having an extremely high monster generation rate, and nowhere to run to.
The Si level
The Si is an Artifact that is found on level 8 of the Caverns of Chaos. It is guaranteed artifact, generated in every single game. It always located on a trap, and takes on the appearance of a regular light grey tool ("["). It replicates in your backpack and, being an artifact, is both indestructible and very valuable, but has no other practical applications.
See main article: Dwarftown.
Dwarftown is typically found on D:9 to D:11. It is one of the most important locations in the mid-game, due to its guaranteed altar, shop, forge, various quests and training hall, among other things.
See main article: Animated Forest.
Somewhere around D:13, you will encounter two sets of down stairs, one of which will lead to the Animated Forest. This is a room the size of the Big Room mentioned above, however it is filled to the brim with animated trees.
It can be quite a task to get across without angering too many trees, which can also hit quite hard and slowly kill players who enter with too few armor (they do not see in the dark, however). It is generally accepted that while tedious, going through the Animated Forest is much safer than your other option. Avoid combat if you're not a teleport-controlling spellcaster with high PV and HP.
See main article: Dwarven Halls.
On the level leading to the Animated Forest, the second down-stairs lead to the Dwarven Halls.
The Dwarven Halls are a special two-level sub-dungeon with an extremely high danger level, relative to the surrounding area. It is quite common to encounter strong monsters such as quickling queens and occasionally even greater molochs. Invisibility, magic mapping and controlled teleportation can reduce some risk. Seven league boots are quite a boon, too.
Thrundarr will ask you to report to him what lies beyond the animated forest or Dwarven Halls; crossing either will fulfill the quest. Note that entering the Animated Forest for even a single turn before the quest is given will mean you never receive it or its reward.
See main article: Moon Sickle
The Moon Sickle is a guaranteed artifact that is typically found between dwarftown and wall of flames. It is always guarded by a stone golem.
See main article: Khelavaster.
Khelavaster, the dying sage, is always found standing on the down stairs of D:16. He will provide information about the Orbs, and if he can be saved, will open the possibility of Ultra Endings.
See main article: Graveyard.
The reward for digging up the many graves (though a chaotic act) can be large, assuming you can deal with whatever lay buried. Especially rewarding, and appropriately dangerous, are the "Great treasures are buried here" graves. It is advised to have death resistance and emergency teleportation available in case you have to face an emperor lich, or lich king, for that matter.
The stairwell leading down from the graveyard is covered by a dwarven portal. A lever protected by Thrundarr in Dwarftown can open it. Thrundarr will open the portal for you if you complete the Portal Quest.
The Water Temple
See main article: Water Temple.
The Water Temple is first of the elemental temples in the Caverns of Chaos, typically found around D:19. The temple itself is disconnected from the rest of the level, so some means of digging is required to gain entry. Note that teleportation does not work on the Water Temple level.
The Banshee level
See main article: Banshee.
The Banshee level is found around D:22. It houses the only Banshee naturally generated by the game (more can be wished for).
After the banshee level, and before the wall of flames, the numeric level order gets slightly confused; some levels will be skipped and entered later.
The Wall of Flames
See main article: Wall of flames.
The Eternal Guardian
See main article: Eternal guardian.
The eternal guardian can be found around level D:30. He will only let players with the Ring of the High Kings pass, although there are ways to get past him without the ring.
See main article: Casino.
The Casino can typically be found around D:31, after the eternal guardian. It features the biggest shop in the game, as well as nearly 250 slot machines which allow the PC to gamble.
Around D:32, there will be a level with two sets of stairs up. They lead to D:31 (where you came from), and Darkforge. Once you have gone up the Darkforge Shortcut, you can go back down, however you cannot go down it unless you have been up at least once. Bypassing the eternal guardian without giving him the ring will cause him to block the way down, making this a one-way exit; he must be bypassed again if one is to return to the lower parts of the Caverns of Chaos.
Battle Bunny level
Around D:33, you will encounter a level shaped like a lying "U". On the bottom half of the level are many Battle Bunnies and a single bunny master named Bugs. Battle Bunnies breed at an exponential rate, so by the time you get to them in the natural way there will likely be very many of them, and if you take too long to get there and wipe them out, they may start breeding faster then you can kill them and fill the level completely. It is very advisable to keep this from happening by teleporting or digging directly to the bunny master and exterminating him and his dreadful abominations.
See main article: Cat Lord.
On D:35 or D:36, you can find the Cat Lord. He will reward players who have not killed any cave tiger, cave lion or wild cat with the Ring of the Master Cat. Should you have killed cats, however, the Cat Lord will be hostile to you - and he is a formidable opponent.
Note that killing the Cat Lord is not mandatory to progressing further in the dungeon. If the down staircase is generated close to the up staircase, players may well be able to descend before the Cat Lord is encountered. Indeed, given how dangerous the Cat Lord is, players are highly advised to take such an escape if the opportunity presents itself.
See main article: Air Temple.
The Air Temple can be found around D:38. Like all elemental temples, it can only be reached by digging with a wand of digging, Pickaxe, class powers or a spell. It contains a variety of hostile monsters including air elementals and grues, and Master Summoner.
See main article: Earth Temple.
The Earth Temple is typically found on D:44. It contains many earth elemental and stone grues, which are capable of moving through stone. You will have to face the Ancient Stone Beast to get what you came for.
See main article: Unreal Cave.
See main article: D:48.
D:48 has five anomalies on it, corresponding to the five chaos orbs. When all five orbs have been thrown into their anomaly, the up staircase will turn into a down staircase, allowing access to D:49. Note that once the staircase has been turned around, it is impossible to go back - they will turn back again only if the Chaos Gate has been closed.
See main article: D:49.
D:49 is often called the "frictionless level". It is extremely icy, and impossible to walk across normally. The player must throw objects in the opposite direction to where they want to travel, or kick walls to propel him- or herself.
See main article: D:50.
D:50 is the final level of ordinary games, the home of the Chaos Gate. You must battle through (or cunningly bypass) hordes of powerful monsters to close the gate, protecting Ancardia from the incursions of Chaos.
The Cavern of Chaos has many inhabitants. A summary of special inhabitants of each level can be found on that level's page.
There are several quests associated with the Caverns of Chaos, although not all are listed as such.