Last November, a huge seismic event that shook the planet left experts wondering about its possible source. Researchers now think they know what might have caused it: an offshore volcanic event unlike any other in recorded history. If the hypothesis is right, and there has been a massive movement of magma underneath the sea floor, that has implications for nearby Mayotte and the neighbouring Comoros islands off the coast of Africa. Mayotte has already started to sink (by around 9 mm or 0.35 inches per month) and move eastward (by around 16 mm or 0.63 inches per month) – movements that would tally with an underground chamber getting deflated as magma flows out. "We believe that the 2018 crisis is associated to an eruption, despite the fact that we do not have direct observations so far," write the researchers behind the new study. "It might be the offshore eruption with the largest volume ever documented." Based on the seismic readings taken in the area over six months leading up to the