The possibility of a lake roughly 20 kilometers across under the Red Planet’s southern polar ice cap was raised in 2018, when the European Space Agency’s Mars Express spacecraft detected bright spots on radar measurements, hinting at a large body of liquid water beneath 1.5 kilometers of solid ice that could be home to living organisms. Subsequent work found hints of additional pools surrounding the main lake basin. But the planetary science community has always held skepticism over the lakes’ existence, which would require some kind of continuous geothermal heating to maintain subglacial conditions. Below the ice, temperatures average –68° Celsius, far past the freezing point of water. An underground magma pool would be needed to keep the area liquid — an unlikely scenario given Mars’s lack of present-day volcanism. In a study published in the July 16 Geophysical Research Letters, planetary scientist Carver Bierson of Arizona State University and colleagues describe a couple other substances that could explain the reflections. Radar’s reflectivity depends on the electrical conductivity of the material the radar signal moves through. Liquid water has a fairly distinctive radar signature, but examining the electrical properties of both clay minerals and frozen brine revealed those materials could mimic this signal. Adding weight to the non-lake explanation is a study from an independent team, published in the same issue of Geophysical Research Letters. The initial 2018 watery findings were based on MARSIS data focused on a small section of the southern ice cap, but the instrument has now built up three-dimensional maps of the entire south pole, where hundreds to thousands of additional bright spots appear. “We find them literally all over the region,” says planetary scientist Aditya Khuller.