The Red Sea, now one of the world’s most important marine ecosystems, dried out completely about 6.2 million years ago before being suddenly refilled by a catastrophic flood from the Indian Ocean, according to new research.

Scientists at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia say they have found conclusive evidence that the dramatic transformation happened in less than 100,000 years, an instant in geological terms.

Their study shows that the Red Sea changed from a salt-filled desert basin to a deep marine gulf when seawater burst through volcanic barriers at the Bab el-Mandab Strait and reconnected it with the world’s oceans.

“Our findings show that the Red Sea basin records one of the most extreme environmental events on Earth, when it dried out completely and was then suddenly reflooded about 6.2 million years ago,” said lead author Dr Tihana Pensa of KAUST. “The flood transformed the basin, restored marine conditions and established the Red Sea’s lasting connection to the Indian Ocean.”

A vanished sea returns

The Red Sea was once linked to the Mediterranean through a shallow northern channel. When this closed, the basin dried out, leaving behind thick layers of salt and gypsum. In the south, a volcanic ridge near the Hanish Islands blocked the connection to the Indian Ocean.

Around 6.2 million years ago, that ridge was breached and seawater poured into the basin in a torrent. The flood carved a 320 kilometre-long submarine canyon that remains visible on the seafloor today. Within a short time, the Red Sea was transformed from a barren salt desert into a restored marine environment.

This event took place almost a million years before the well-known Zanclean flood, which refilled the Mediterranean after the Messinian salinity crisis.

A natural laboratory

The Red Sea began to form 30 million years ago when the Arabian Plate separated from Africa. At first it was a rift valley filled with lakes, later flooded by the Mediterranean about 23 million years ago. Early marine life thrived, but rising salinity and poor circulation caused widespread die-offs between 15 and 6 million years ago, culminating in total desiccation.

The catastrophic refilling of the basin reintroduced normal marine conditions and allowed coral reef ecosystems to take hold, many of which continue to thrive today.

“The Red Sea is a natural laboratory for understanding how oceans are born, how giant salt deposits form, and how climate and tectonics interact over millions of years,” said co-author Prof Abdulkader Al Afifi of KAUST.

The findings underline the extreme environmental history of the Red Sea and show how quickly oceans can be reshaped by changes in climate and tectonics.