I looked ahead of me. There was one low ridge of ground between me and the enormous boulders. A neat four-inch semicircle was bitten out of the very top of the ridge. In the house-sized boulder directly beyond was a four-inch hole. * * * * * Allenby winced and called the others when I came back and reported. "The mystery _deepens_," he told them. He turned to me. "Lead on, Peters. You're temporary _drill_ leader." Thank God he didn't say _Fall in_. The holes went straight through the nest of boulders--there'd be a hole in one and, ten or twenty feet farther on in the next boulder, another hole. And then another, and another--right through the nest in a line. About thirty holes in all. Burton, standing by the boulder I'd first seen, flashed his flashlight into the hole. Randolph, clear on the other side of the jumbled nest, eye to hole, saw it. Straight as a string! The ground sloped away on the far side of the nest--no holes were visible in that direction--just miles of desert. So, after we'd stared at the holes for a while and they didn't go away, we headed back for the canal. "Is there any possibility," asked Janus, as we walked, "that it could be a natural phenomenon?" "There are no straight lines in nature," Randolph said, a little shortly. "That goes for a bunch of circles in a straight line. And for perfect circles, too." "A planet is a circle," objected Janus. "An oblate spheroid," Allenby corrected. "A planet's orbit--" "An ellipse." Janus walked a few steps, frowning. Then he said, "I remember reading that there _is_ something darned near a perfect circle in nature." He paused a moment. "Potholes." And he looked at me, as mineralogist, to corroborate. "What kind of potholes?" I asked cautiously. "Do you mean where part of a limestone deposit has dissol--" "No. I once read that when a glacier passes over a hard rock that's lying on some softer rock, it grinds the hard rock down into the softer, and both of them sort of wear down to fit together, and it all ends up with a round hole in the soft rock." "Probably neither stone," I told Janus, "would be homogenous. The softer parts would abrade faster in the soft stone. The end result wouldn't be a perfect circle." Janus's face fell. "Now," I said, "would anyone care to define this term 'perfect circle' we're throwing around so blithely? Because such holes as Janus describes are often pretty damned round." Randolph said, "Well...." "It is settled, then," Gonzales said, a little sarcastically. "Your discussion, gentlemen, has established that the long, horizontal holes we have found were caused by glacial action." "Oh, no," Janus argued seriously. "I once read that Mars never had any glaciers." All of us shuddered. * * * * *