Question

A sound wave traveling downward with a speed of about 4,000 m/s suddenly slows to 1,500 m/s not far below the Earth’s surface.

What has the sound wave most likely encountered?

a different kind of rock
an underground lake
an open cavity
a pile of dinosaur bones

Answer 1

An underground lake.

Reason: Typical sound (seismic P‑wave) speeds in solid rock are around a few thousand m/s (~4,000 m/s), while the speed in water is about 1,500 m/s. An open air cavity would give a much lower speed (~340 m/s), and a pile of bones or a different rock type would not drop the speed that far.