NASA's Perseverance Mars rover has detected underground remains of an ancient river delta on Mars, some of the oldest evidence yet obtained showing how water once flowed on the surface of Earth's planetary neighbor. The rover revealed geological features up to 115 feet underground while exploring Jezero Crater, an area in the Martian northern hemisphere believed to have been flooded with water. The researchers identified layered sediments and eroded surfaces indicative of a delta environment, a large-scale fan-shaped deposit of sediment formed at the location where a river entered a larger body of water. They estimated the now-buried delta dates to about 3.7 to 4.2 billion years ago. The findings were based on RIMFAX's deepest data collected from September 2023 to February 2024 over a span of 250 Martian days.