Scientists believe Mars may not have always been the cold, dry planet we see today. A new study suggests that billions of years ago, a massive ocean could have covered nearly one-third of its surface.

The strongest clue comes from what researchers describe as a “bathtub ring” — a wide, flat band of land that may mark the edge of an ancient ocean. This feature is similar to the continental shelves found on Earth, which form where land meets ocean.

To test their theory, scientists ran simulations showing how oceans disappear over time. Their results revealed that continental shelves remain one of the most stable and visible features, even after water vanishes. When they examined Mars data, they found signs of a similar structure, although not identical to Earth’s version.

The idea of water on Mars is not new. Previous missions have already discovered dried riverbeds, lake remnants, and mineral deposits that form in the presence of water. However, whether Mars once had a large, long-lasting ocean has remained a topic of debate among scientists.

Earlier evidence, such as suspected shorelines, has been questioned because the elevations were inconsistent. The newly proposed coastal shelf offers a more reliable indicator because it is wider, more durable, and less likely to be erased over time.

Additional support comes from other discoveries, including underground signs of ancient beaches and sediment deposits likely formed by flowing water. These findings suggest Mars once had an active water cycle, possibly including rivers feeding into a large ocean.

Despite these clues, scientists caution that the evidence is not yet definitive. Mars differs significantly from Earth, especially because it lacks plate tectonics — a key process that shapes Earth’s oceans and continents. This makes direct comparisons challenging.

Future missions could provide clearer answers. The upcoming European rover, Rosalind Franklin, is expected to explore regions where this ancient ocean may have existed, using advanced tools to analyze both surface and subsurface materials.

Confirming the existence of a Martian ocean would be a major breakthrough. It could reshape our understanding of the planet’s climate history and strengthen the possibility that Mars once supported life.

For now, the mystery remains open — but with each discovery, scientists are getting closer to answering one of humanity’s biggest questions: Was Mars ever truly a living world?