Erosion on Mars is exposing deposits of water ice, starting at depths as shallow as one to two meters below the surface and extending 100 meters or more. The ice is a critical target for science and exploration: it affects modern geomorphology, is expected to preserve a record of climate history, influences the planet's habitability, and may be a potential resource for future exploration. Whilst water ice is known to be present in some locations on Mars, many questions remain about its layering, thickness, purity, and extent. Now, Colin Dundas and colleagues have pinpointed eight locations, using the Mars Reconnaissance Orbiter (MRO), where steep, pole-facing slopes created by erosion expose substantial quantities of sub-surface ice. The fractures and steep angles indicate that the ice is cohesive and strong, the authors say. What's more, bands and variations in color suggest that the ice contains distinct layers, which could be used to understand changes in Mars' climate over time (the ice sheets themselves likely formed as snow accumulated over time). Since there are few craters on the surface at these sites, the authors propose that the ice was formed relatively recently. Images taken over the course of three Martian years reveal massive chunks of rock that fell from the ice as erosion occurred, leading the researchers to estimate that the ice is retreating a few millimeters each summer. Because the ice is only visible where surface soil has been removed, Dundas et al. say it is likely that ice near the surface is even more extensive than detected in this study. The ice could be a useful source of water for future missions to Mars.