The map is entirely made up of random terrain types. Every turn about 1/3 of the terrain types are re-randomized, possibly causing the death of monsters by drowning or imprisoning in rock. Appearing mountains will not kill players, but other terrain might. Players are advised to bring along a grappling hook (for escape from quicksand/mud), Skidbladnir (for water), and a source of fire resistance (for lava).
Note that monster generation in Chaos may include any monster in the game, including some very strong ones. Be ready to leave on a moment's notice.