The history of water on Mars flows a lot deeper than scientists once believed. A new project has mapped hundreds of thousands of rock formations on the Red Planet that may have been altered by large amounts of water in the past.

Mars is a dry planet today, but various evidence suggests it once had flowing water across its surface. Aqueous minerals can be found in rocks that were chemically altered by water in the past, and typically turned into clays and salts. When small amounts of water interact with the rocks, they remain relatively unchanged and retain the same minerals found in the original volcanic rocks. But if large amounts of water interact with the rocks, then soluble elements are dissolved by the water, leaving behind more aluminum-rich clays.