Question

When the scientists drilled into the ground, they found a system of subterranean rivers.

Based on the context, subterranean means which of the following?

Answer 1

subterranean: "existing, occurring, or done under the earth's surface."

Answer 2

Sub like as in "submarine", sub being 'under' and marine meaning 'water'........
Terra is the Earth's name.
So subterranean obviously means "The Earth's name is under water"!