Researchers reviewed ocean floor samples collected during the Deep Sea Drilling Project in 1975. Credit: Deposit Photos / Oleg Dorokhin Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent every weekday. There was a time long ago when the Atlantic Ocean didn’t exist. The general understanding among geologists is that the body of water originated between 83 to 113 million years ago, when South America and Africa split into their two respective continents to form the Equatorial Atlantic Gateway. However, Earth’s marine history appears to require a multimillion-year revision thanks to a recent discovery roughly half a mile beneath the ocean floor. The evidence is explored in a study published in the June edition of the journal Global and Planetary Change. According to geologists at the UK’s Heriot Watt University, gigantic waves of mud and sand sediment about 250 miles off the coast of Guinea-Bissau in West Africa indicate the Atlantic Ocean actually formed around four million years earlier than previous estimates. To understand just how intense all of this movement was, imagine waves that are about half a mile long and over 300 feet high.  “A whole field formed in one particular location to the west of the Guinea Plateau, just at the final ‘pinch-point’ of the separating continents of South America and Africa,” study co-author Uisdean Nicholson explained in a statement. Nicholson and their colleagues initially came across these layers of mud waves after comparing seismic data with core samples collected from wells during the Deep Sea Drilling Project (DSDP) of 1975. Five layers in particular were utilized to recreate the tectonic processes that broke apart the ancient supercontinent of Gondwana during the Mesozoic Era. “One layer was particularly striking: it included vast fields of sediment waves and ‘contourite drifts’—mud mounds that form under strong bottom currents,” said Nicholson. These waves initially formed as dense, salty water poured out from the newly created Equatorial Atlantic Gateway, “like a giant waterfall that formed below the ocean surface,” he added. Just before the geologic event, huge salt deposits formed at the bottom of what is now the South Atlantic. After the gateway opened, the underground mudfall occurred when dense, relatively fresh Central Atlantic water in the north combined with very salty waters in the south. The resulting sedimentary evidence examined by the study’s authors now indicates this opening seems to have started closer to 117 million years ago. “This was a really important time in Earth’s history when the climate went through some major changes,” explained study co-author Débora Duarte. “Up until 117 million years ago, the Earth had been cooling for some time, with huge amounts of carbon being stored in the emerging basins, likely lakes, of the Equatorial Atlantic. But then the climate warmed significantly from 117 to 110 million years ago.” Duarte and Nicholson believe part of that major climatic change  helped from the Atlantic Ocean, as seawater inundated the newly formed basins. “As the gateway gradually opened, this initially reduced the efficiency of carbon burial, which would have had an important warming effect,” said Duarte. “And eventually, a full Atlantic circulation system emerged as the gateway grew deeper and wider, and the climate began a period of long-term cooling during the Late Cretaceous period.” The ramifications go beyond revising Earth’s geological timeline or the gateway’s role in Mesozoic climate change. Better understanding the influence of oceanic evolutionary journeys on ancient climate patterns can help to predict what the future holds for the planet.  “Today’s ocean currents play a key role in regulating global temperatures,” explained Nicholson. “Disruptions, such as those caused by melting ice caps, could have profound consequences.”