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.

* * * * *

Half an hour later, we spotted more holes, about a mile down the
'canal,' still on a line, marching along the desert, through cacti,
rocks, hills, even through one edge of the low vegetation of the 'canal'
for thirty feet or so. It was the damnedest thing to bend down and look
straight through all that curling, twisting growth ... a round tunnel
from either end.

We followed the holes for about a mile, to the rim of an enormous
saucerlike valley that sank gradually before us until, miles away, it
was thousands of feet deep. We stared out across it, wondering about the
other side.

Allenby said determinedly, "We'll burrow to the _bottom_ of these holes,
once and for all. Back to the ship, men!"

We hiked back, climbed in and took off.

At an altitude of fifty feet, Burton lined the nose of the ship on the
most recent line of holes and we flew out over the valley.

On the other side was a range of hefty hills. The holes went through
them. Straight through. We would approach one hill--Burton would
manipulate the front viewscreen until we spotted the hole--we would pass
over the hill and spot the other end of the hole in the rear screen.

One hole was two hundred and eighty miles long.

Four hours later, we were halfway around Mars.

Randolph was sitting by a side port, chin on one hand, his eyes
unbelieving. "All around the