The phrase "deep caves" refers to underground spaces that are very far beneath the surface of the Earth. These caves usually have long tunnels, large chambers, and are often very dark and difficult to explore. Full definition
I've seen photos of trees growing in deep cave - collapse sinkholes that got no direct sun. (realclimate.org)
Gather your strength and your courage, and invite your friends to embark on an epic fantasy LEGO adventure through the darkest forests and deepest caves of Middle - earth! (cosmosgaming.com)
Keep an eye out for jewfish and other large pelagics that visit these reefs to rest in the many deep caves found toward the base of the channels along the reef front. (ambergriscaye.com)