He has yet to treat/color their surfaces. The cavern will be pigmented black, so it will be difficult to judge its depth. The sculpture will be a weird iridescent green that changes as you walk around it.
It reminded me of this Rebecca Solnit quote in which she describes a cave:
All forms were jumbled together in this underworld, as though this were a place where all the patterns for the surface of the earth were worked out, like a laboratory or a mine from which not materials but shapes and textures came. This sense of a raw, primordial workshop was enhanced by the lack of color. Other than the reddish formation in one of the shallower caves, there was no color but the slight variety from alabaster to amber to brown, with the deep shadows cast by the lights and the depths beyond the lights an inky black. But scale, texture, shape, depth, height, nearness, and distance were all there in infinite variation. There were places where the rock was so gleaming it looked like it was slimy or was a pile of slime. Sometimes the shallow pools reflected the stone above so that it seemed to reach far down into the depths, and a few of the depths fell into darkness that might have gone on forever. The forms were strangely stirring, both erotic and terrifying. Some places had flat crusts through which time and force had broken.”