At the beginning of The Wine Dark Sea, the Captain and his crew experience a rain of volcanic bombs. This natural phenomenon occurs from volcanoes that arise at subduction zones. As the movement of the tectonic plates forces one plate under another, most often a sea floor under a continental plate, volcanoes form. In an area where the ocean floor, with its sediments and decayed organisms (rich in water and carbon) are forced downwards, they become super-heated and volcanoes form to release the pressure of the steam and other gases. The gases, mixed with molten lava, make explosive eruptions and, sometimes, form volcanic bombs.
These particular bombs are from California, at the other side of the Pacific Ocean that produced the attack on the Captain. So many of the Doctor's observations of natural phenomena were at sea, we should cover some of those.