"The Martians lived on the surface long ago, in those cities that are still there," Susan explained. "Dad studied them a long while and said they're partly like plants, but with blocked electronic and electrostatic charges in their systems that even he didn't pretend to understand. "We learned all this bit by bit. Metals have always been scarce on Mars, so the Martians concentrated on biological engineering instead of mechanics, breeding special creatures to fit their needs. Those are _voras_, their living tools and servants and clothes and weapons. That _varlu_ is just a specialized _vora_. They respond to thought waves and Martians can control them from quite some distance. Klev taught us a little about them, but human thought waves are of a different pattern and I have to actually touch them. Like that screen-vora back in the city." "Can you--?" Nick interrupted. * * * * * Susan shook her head. "No. _Varlus_ answer only to their owners, and even another Martian couldn't pass that one without Merlo's consent. "Seven or eight centuries ago," she continued. "A spaceship crashed on Mars. Dad believed it came from clear outside this solar system. All the creatures inside were dead when the Martians reached the crumpled hull. "It brought the Plague. Shortly afterward Martians began to turn blue and shrivel and die. For a while they thought water had something to do with the disease, so they developed huge water-_voras_ that could tunnel through solid rock and pump water, and they drained all the surface water down into caverns deep inside the planet. But still the infection spread. "Finally they discovered that sunlight and the Plague were connected, so they abandoned the surface cities and had their _voras_ carve out this great system of tunnels. The plan worked, somewhat. Darkness stopped the spread of the disease. "But Martians are partly plants. Without sunlight they die just as surely as though killed by the Plague. So for the last several hundred years they have barely existed in a precarious balance between the Plague and sunlight starvation. "Nick, they're a doomed race. In the year Dad and I've been here we have seen only two Martian children." "But Klev?" "Yeast. Just plain yeast. I'd brought one package in the Trailblazer, for cooking. But there is no more yeast on Mars." "About the water?" Nick asked. "How come your father didn't tell Harmon about that? Colonel Hammer had us drilling all over the planet." "Luck," Susan replied seriously. "We didn't know where it was ourselves until after we came back from Earth, after we learned more of their language." "But all our drilling," Nick protested. "Surely at least one--"