Debug mode is the master cheat in Carnivores, Carnivores 2, and Carnivores Ice Age. In the first game, it is entered with the key sequence D+E+B+U+G+O+N ("debugon" or "debug on"), whereas the latter two games use D+E+B+U+G+U+P ("debugup" or "debug up"). In all three games, entering the cheat a second time deactivates it.
Once entered, debug mode has the following effects on the hunter:
invincibility to drowning or lava (dinosaurs can still kill the hunter)
partial invisibility - herbivores will still sense the hunter and run away if you get too close to them, and carnivores sense the hunter but don't really react to you
In addition, debug mode allows the other cheats in Carnivores to be activated. These cheats require debug mode to be activated first in Carnivores 2 and Carnivores Ice Age, as well, but cheats new to these games do not.