This map is a test map that I was working on, it is a single player map and just so happened to mess around with the gravity command in the worldspawn properties and I also played with the triggering effect so when you kill an enemy another will respawn

unrar into your Doom3\base directory
to run start Doom 
1. bring the console down by pressing ctrl + alt + ~
2. then type in map antigravityroom
when the zombie kicks the barrels they should become weightless also when you jump you become weightless
if it doesn't work repeat steps 1 and 2
hint: you may want to put god mode on (in console type god and give all gives you all weps, health, ammo, and shields)
you have a buddy to your left he has something for you at his feet
the enemy will respawn 3 times, there are 3 enemy, but there is a surprise when you kill the last of either one of the enemies