In 2018, Brazilian and British scientists were exploring the seafloor around a volcanic plateau known as the Rio Grande Rise when they spotted rocks that looked like they belonged on dry land. In a recent study, the team showed that the clay's distinctive mineral makeup could have formed only by open-air weathering in tropical heat and humidity. It's the latest in a string of discoveries hinting that this patch of ocean, 1,200 kilometers (750 miles) from Brazil's coast, was once an island