NASA's Mars Orbiter has been mapping vast regions where the Martian landscape is riddled with underground, frozen ice deposits that look very much like the legendary canals of Mars.
According to NASA:
The subsurface ice deposits extend for hundreds of kilometers, or miles, in the rugged region called Deuteronilus Mensae, about halfway from the equator to the Martian north pole . . . "We have mapped the whole area with a high density of coverage," [NASA's Jeffrey] Plaut said. "These are not isolated features. In this area, the radar is detecting thick subsurface ice in many locations." The common locations are around the bases of mesas and scarps, and confined within valleys or craters.
Plaut said, "The hypothesis is the whole area was covered with an ice sheet during a different climate period, and when the climate dried out, these deposits remained only where they had been covered by a layer of debris protecting the ice from the atmosphere."
In the nineteenth century, when astronomer first observed Mars through telescopes, the distant surface of the planet appeared to be riddled with so-called canals that some believed were a water system. Here is a map of the planet from that era.
The idea that Mars was covered in watery canals persisted until the mid-twentieth century, when Ray Bradbury speculated about what the canals might be in his collection The Martial Chronicles. Satellites to the planet have never found any canals. The closest we've come are these ice deposits.
via NASA JPL