The ocean covers more than 70 percent of the surface of Earth today, but over 3 billion years ago the entire planet may have been covered in water. 

To investigate what our planet was like billions of years ago, a team of researchers turned to indirect methods of analysis at a geological site in the Australian Outback known as Panorama in the Pilbara Craton. Today, the region is 100 kilometers (62 miles) inland, but 3.2 billion years ago, it was deep underwater. In fact, there were also once ancient hydrothermal vents there where seawater bubbled up through the ocean floor.

"There are no samples of really ancient ocean water lying around, but we do have rocks that interacted with that seawater and remembered that interaction," lead author Benjamin Johnson, an assistant professor at Iowa State University, said in a statement.

The researchers collected over 100 of these rock samples and found something peculiar in them: a slightly higher amount of a particular isotope of oxygen. An isotope is a version of an element that has a different number of neutrons in its nucleus. The vast majority of oxygen on Earth is Oxygen-16 and contains eight protons and eight neutrons. However, about 0.20 percent of the planet’s oxygen consists of 10 neutrons and is known as Oxygen-18.  

Samples from Australia show that the region’s ancient water contained more Oxygen-18 than present times. Since continents are covered in clay-rich soil that is efficient at trapping Oxygen-18, the team suggests that without continents covered in soil, more Oxygen-18 would have been present in the ocean. Their discovery indeed suggests there wasn't much dry land available 3.24 billion years ago. 

"There's nothing in what we've done that says you can't have teeny, micro-continents sticking out of the oceans," added co-author Boswell Wing, an associate professor in the Department of Geological Sciences at the University of Colorado Boulder. "We just don't think that there were global-scale formation of continental soils like we have today."

The researchers don’t know when and how continents began to emerge out of the ocean but they plan to investigate “younger” rock formations to piece together a more precise timeline. The findings are reported in Nature Geoscience.