Because you can't see through it. since it's impossible to see through it it's equally impossible
to see the bottom of the cave or know how large it is. Where it's almost completely full it is even
impossible to know where the cave is or its general direction.
All we can see is the top of part of the cave and even it isn't well mapped or understood. The
bottom of the cave will be more interesting because it will go to more places and it will have more