Barely a year ago, Mars Odyssey found signs that the planet has reservoirs of underground ice near the south pole. Scientists at the US space agency (Nasa) estimated there was enough ice to fill Lake Michigan twice. They said it might be merely the tip of the iceberg and it seems they were right. New observations by Mars Odyssey and Mars Global Surveyor (another Nasa probe that is mapping Mars) suggest the planet’s north pole has about one third more underground ice than the south. Beneath a shallow crust of dry soil, there appears to be a layer of permanently frozen ground that is up to 75% ice.