They were looking for the bill, but they were not conventional because it had been thousands of years since the last one in the region, and it was not seen at the surface.
Researchers have recently mapped the seabed in the area, reports Science magazine, and they have confirmed the cause – a brand new volcano, 800m tall and five kilometers wide.
"We have never seen anything like this," said Nathalie Feuillet, expedition leader.
About five cubic kilometers of magma erupted onto the seabed, the French team found. Rocks brought to the surface popped open once they were brought on board, releasing high pressure gasps when the magma cooled in the deep ocean.
But why did an eruption happen now? Africa itself is splitting up thanks to continental drift, which could be behind the renewed activity, one geologist told Science.
A magma chamber is suspected to be shrinking as a result, pushing its contents upwards towards the surface and the land above the subside. The island of Mayotte has sunk 13cm and shifted 10cm east in the past year as a result.
Since the volcano's birth, earthquake activity in the region, and to appear to be trending towards the island, raising fears of a tsunami.