A seismic mystery begins
In May 2018, the French island of Mayotte in the Indian Ocean began to shake. The area, previously considered volcanically dormant for over 4,000 years, experienced thousands of earthquakes over several months. The swarm included the largest earthquake ever recorded in the Comoros archipelago, a magnitude 5.9 event on May 15, 2018, which caused damage to several buildings. This unprecedented seismic activity puzzled scientists and alarmed the island's residents.
The mystery deepened on November 11, 2018. Seismometers across the globe, some nearly 18,000 kilometers away, detected a strange and powerful signal originating from off the coast of Mayotte. It was a very-low-frequency hum, almost a pure monochromatic wave that repeated every 17 seconds. The vibration, which lasted for over 20 minutes, rang through the planet, yet no human directly felt it. Unlike typical earthquakes that generate sharp P-waves and S-waves, this event was composed almost entirely of long-period surface waves, a sign often associated with volcanic processes.
The hunt for the source
The combination of the earthquake swarm and the bizarre global hum spurred an immediate scientific investigation. GPS stations on Mayotte revealed another startling fact: the entire island was moving. It was steadily shifting eastward and sinking. By December 2021, the island had moved 21 to 25 centimeters to the east and subsided by 10 to 19 centimeters. The movement suggested a massive deflation of a magma chamber deep beneath the seafloor.
In May 2019, an oceanographic mission named MAYOBS1 surveyed the seabed 50 kilometers east of Mayotte. Using multibeam echo sounders, researchers mapped the area where the seismic activity was concentrated. They discovered something astonishing: a new submarine volcano had formed on the ocean floor, 3,500 meters below the surface. The newly christened Fani Maoré volcano stood 820 meters tall, with a base 4 to 5 kilometers in diameter. The eruption that formed it is considered the largest documented submarine eruption in history, having expelled more than 5 cubic kilometers of lava. Scientists determined the magma originated from an exceptionally deep reservoir, between 30 and 50 kilometers down in the Earth's mantle. The draining of this deep chamber caused the island's sinking and eastward drift. To monitor the ongoing activity, the French government established the Mayotte Volcanological and Seismological Monitoring Network (REVOSIMA).