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: an underground lake

In general, sound (mechanical waves) travels faster in solids than in liquids, and faster in liquids than in gases. This is because the speed of the mechanical waves is determined by a relationship between the elastic properties of the medium in which they are propagated and the mass per unit volume of the medium (that is:density).

In other words: The speed of sound varies depending on the medium through which the sound waves travel.

So, if we are told the sound wave initially had a speed of 4,000 m/s and it suddenly decreases to 1,500 m/s, this means the sound waves passed from a solid medium to a liquid medium.

Hence, the correct option is: an underground lake.

an underground lake