The Underground Facility is an underground area in Tokyo Jungle. It is unlocked by completing story mission 13 and retrieving the USB from Yoyogi Park East. The Underground Facility is where the last story mission, Act. 14: Tokyo Jungle, is set.
The Underground Facility is presumably where scientists researched the time machine that caused the people of Tokyo to disappear.
If you enter the Underground Facility before the sixtieth year but after the tenth so that it will still be populated, aside from being packed with ERC-2000s, this is an easy way to spot Saber-Toothed Tigers, Deinonychus, Mammoths, Giant Moas, Archaeopteryx, and the occasional Dilophosaurus. In the airway you can also commonly find Porcupines, "Wolfs", and in the spot between the airway and the miniboss stretch where you must fight the ERC-2000s wearing the Guard Dog Shoes, Guard Dog Collar, and Guard Dog Uniform, you can find Tigers and mini Crocodiles.
No events usually occur in the Underground Facility.