cav·ern
/ˈkavərn/
noun
a cave, or a chamber in a cave, typically a large one.
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground.
used in similes and comparisons to refer to a vast, dark space.
“the dark cavern of the main performance hall”