NASA's Curiosity Rover has discovered opals — gemstones formed by silica alteration by water — on the Red Planet's Gale Crater.
Arizona State University researchers noticed fracture halos — rings of light-toned rocks — by analyzing data and images beamed back by NASA's Curiosity Rover, which has been exploring the Crater since 2012. Further tests confirmed the opal-rich composition of these fracture halos.
As ice has already been found at the poles of Mars, the discovery that water was once within these immensely dry craters proves the planet was undoubtedly a much wetter place a long time ago. Water facilitates life on Earth, and if more of it is found in these crater basins, Mars might one day make for a habitable environment for human beings.
Though it's a possibility, the idea of finding life on Mars, let alone the ability of humans to live there one day, is a big stretch. Even if vast amounts of water and life are found in the planet's subsurface areas, it's because they are also shielded from the Sun's dangerous radiation — something surfaces above can't be protected from due to Mars' thin atmosphere.