Despite claims in the 1890s that Mars was filled with canals teeming with water*, research over the past several decades has suggested that in fact, Mars has only a tiny amount of water, mostly near its surface. Then, during the 1970s, as part of NASA’s Mariner space orbiter program, dry river beds and canyons on Mars were discovered—the first indications that surface water may have once existed there. The Viking program subsequently found enormous river valleys on the planet, and in 2003 it was announced that the Mars Odyssey spacecraft had actually detected minute quantities of liquid water on and just below the surface, which was later confirmed by the Phoenix lander.Surprisingly, the evidence used to reach this conclusion comes from analysis of "the amount of water molecules locked inside crystals of the mineral apatite" in meteorites that originated from Mars. Discussion at the link.
Now, according to an article published [June 21] in the journal Geology, there is evidence that Mars is home to vast reservoirs of water in its interior as well. The finding has weighty implications for our understanding of the geology of Mars, for hopes that the planet may have at some point in the past been home to extraterrestrial life and for the long-term prospects of human colonization there.
* Can something "teem" with water? (And "amount of water molecules" is also awkward. Better copyediting needed).