In that case, you can see that although the walls are crumbly black stone covered in blue-green, glowing algae, the floor is formed of milk-white stone blocks.

You break through the surface in a small, dimly lit cave, with a sloping white sand beach. The source of the light is a brilliantly glowing, foot-high spool of fine silver thread. The cave is otherwise empty except for a doorway obscured by a thick grass curtain.