e, the edges of the groove began to curve inward
over the groove. They touched. No more groove.

The ground had risen, the groove had stayed level and gone underground.

Except that now it wasn't a groove. It was a round tunnel.

A hole.

A few paces farther on, I thumped the ground with my heel where the hole
ought to be. The dirt crumbled, and there was the little dark tunnel,
running straight in both directions.

I walked on, the ground falling away gradually again. The entire process
was repeated in reverse. A hairline appeared in the
dirt--widened--became lips that drew slowly apart to reveal the neat
straight four-inch groove--which shrank as slowly to a shallow line of
the ground--and vanished.

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