Ocean floors rarely linger on Earth's surface—at least in geological time. Most ocean crust is 200 million years old or younger; anything older has been dragged into the mantle by the subduction of plate tectonics. But geologists have suspected that a stretch of the eastern Mediterranean Sea between Cyprus, Crete, and Egypt (pictured) might prove an exception, a bit of ancient ruin amid the bustling oceanic present. Now, this region, called the Herodotus Basin, has been shown to be 340 million years old—the oldest ocean crust on Earth that remains under the sea, according to a study published online today in Nature Geoscience . In 2012 and 2014, researchers dragged a sensitive magnetometer behind the RV Mediterranean Explorer, gathering 7000 kilometers of data in long parallel tracks, when combined with a previous 1998 expedition. The signatures they picked up resembled the striped patterns seen at midocean spreading centers, where alternating magnetic orientations are imprinted on new patches of oceanic crust as Earth’s magnetic field flip-flops over time. Comparing changes in these magnetic stripes with known patterns of the African plate during its long northeastern migration, the researchers estimate that the Herodotus Basin is 315 million to 365 million years old. It was likely part of the Tethys Ocean, the researchers say, an ancient sea from the time of the Pangaea supercontinent.