Ground-penetrating radar on board NASA’s Mars Perseverance rover has confirmed that the Jezero Crater, formed by an ancient meteor impact just north of the Martian equator, once harboured a vast lake and river delta. Over eons, sediment deposition and erosion within the crater shaped the geologic formations visible on the surface today. The discovery of lake sediments reinforces the hope that traces of life might be found in soil and rock samples collected by Perseverance. The crater filled with water has layers of sediments deposited on the crater floor. The lake subsequently shrank and sediments carried by the river that fed it formed an enormous delta. As the lake dissipated over time, the sediments in the crater were eroded, forming the geologic features visible on the surface today. Perseverance’s soil and rock samples will be brought back to Earth by a future expedition and studied for evidence of past life. Between May and December 2022, Perseverance drove from the crater floor onto the delta, a vast expanse of three-billion-year-old sediments that, from orbit, resembles the river deltas on Earth.