Sound waves are both compressional and longitudinal, (although we frequently draw them as transverse waves).
Longitudinal waves are where the particles of matter collide into one another, pushing them into the next and then oscillating back. This is how sound travels. This is also why sound can only travel through a medium, it cannot travel through a vacuum, as is shown here:
Because we can’t tunnel down, so in order to study the interior layers, we must use indirect methods.
