"It was not an easy field campaign," said Gérard Zinzindohoué of the Institut de Recherches Halieutiques et Océanologiques du Bénin, lead author of the article in Frontiers in Marine Science. "It was a mix of long days at sea, technical problems and a lot of uncertainty. But then we saw structures on the sonar that looked like they could be reef signatures, which gave us hope that we were close.

"Next, we surveyed the structures with an underwater drone and the Deep Sea Camera System. That was a big moment for us, because suddenly we could actually see the seafloor. I still remember seeing those first images. It was a mix of excitement and disbelief, because after all this time thinking about this reef, suddenly there was something real in front of us."

The unexpected survivor

The historical surveys had identified a possible reef more than 50 meters (164 feet) beneath the surface and declared that it was probably dead. Zinzindohoué and his colleagues reasoned that it could have been a mesophotic coral ecosystem.

"A mesophotic coral ecosystem lives deeper than the usual shallow coral reefs, in zones where light is much more limited," explained Zinzindohoué. "Unlike typical shallow reefs, it does not always form a single continuous reef structure. Instead, it can appear as patchy or scattered coral communities spread across the seafloor."