ANCIENT fluvial networks on the surface of Mars suggest that it was warm and wet over three billion years ago. Surface features resembling massive outflow channels provide evidence that, even more recently, the martian crust contained the equivalent of a planet-wide reservoir of water several hundred metres deep(1,2). But arguments based on the isotopic fractionation(3,4) and present-day escape rate of hydrogen in the martian atmosphere require only 0.5 metres of crustal water today and about six metres in the past(5). An additional constraint on the evolution of the isotopic composition of martian water has recently been obtained(6) from measurements of the deuterium to hydrogen ratio of hydrous minerals in the SNC meteorites-meteorites that almost certainly originated on Mars. Here I show that these new data require that the modern crustal reservoirs of martian water must be quite large, at least several metres in global-equivalent depth. The deuterium enrichment of the present martian atmosphere then implies that the reservoir of crustal water on ancient Mars was several hundred metres deep, consistent with the geological evidence(3,4).