From Dungeons of Dredmor Wiki

A special room in the Dungeons of Dredmor, filled with a large number of a wide variety of monsters, including bosses. Once discovered, the room is marked with light blue on the Map.

After opening the room a giant "MONSTER ZOO" is shown on the screen and a counter appears on the top right, underneath the map, showing how many monsters are left. Killing all of the monsters in the Monster Zoo will result in the achievement "Bloody Mess" and will often reward the player with a random artifact.

Monster Zoos do not happen on level 1.

Changing floors before completing a Monster Zoo will prevent you from getting the artifact reward. Similarly, entering a Mysterious Portal prior to completing the zoo will also prevent you from getting the artifact reward.

[edit] Bugs

• Changing floors or quitting and reloading the game before a Monster Zoo has been completely obliterated will result in a bug where the counter will stop counting down. This will also prevent the achievement from being awarded as well. If you enter a shop before the countdown ends, this problem seems to occur sometimes also.
• If two monster zoos generate on a floor, opening one can cause the other to be considered open, that zoo can be seen on the map and all the monsters inside it will be visible until you get close enough to it. The "monsters left" counter will be the total of the two.