Which of the following is the best indication that limestone caves could be present at depth?

A) springs
B) cliffs and overhangs along rivers
C) large quartz crystals laying on the surface
D) gently sloping hills
E) limestone pillars and sinkholes

Limestone Caves

Natural underground spaces formed by the dissolution of limestone by acidic water.

Sinkholes

Depressions or holes in the ground caused by the collapse of a surface layer, often due to the dissolution of underlying limestone.

Quartz Crystals

A common and varied mineral species made of silicon dioxide (SiO2), noted for its hardness and crystalline structure.