Sunken Hollow

The Underground Cave is an area introduced in Monster Hunter 4. It is a large, subterranean cave system that features lakes of green fluids and giant cobwebs,connected to a dormant volcano. When the volcano in the area becomes active and erupts, it'll cover the area in lava and change this area to the Underground Volcano. This area is home to many monsters which are suited to life in a dark, wet environment, such as  Khezu, Gypceros, Nerusukyura, and Tetsukabura.