urprised. "Why ... I hadn't thought of it--but it could have
been. That valley was so isolated and deserted, probably nobody would
ever have spotted it.

"Right," Haines added, "and our investigation team studied the remains,
the foundations, the layout, and we're sure it's been there at least
three decades. That's one clue.

"The second clue was the relative flimsiness of the walls. The builders
hadn't expected us to be able to blow them up. They were some sort of
quick construction--a plastic, strong, but not able to hold up against
blasting powder, let alone real heavy bombs or A-bombs.

"Now why was that? And the third clue, why didn't they have a repair
system available, or at least some sort of automatic antiaircraft
defense?"

Burl looked at Ferrati. The latter was watching him shrewdly to see if
he could figure it out.

"The builders didn't expect an air attack," said Burl slowly, "because
of the air disturbances. They did not know we would have a Moon base
that could spot their location. Hence they figured that our civilization
would remain as it was thirty years ago. We wouldn't have been able to
spot the location at that time, because it required outer-space
observation. It might have taken us several years of tramping around to
locate it."

"And the lack of a strong permanent construction? After all, a concrete
and steel-enforced embankment, which any military force on Earth could
have put there, would have balked your dynamite attack," probed Haines.

"That means they didn't have the time or the means to make such a
construction. They must have had a single ship with the kind of
equipment that could lay out a quick base in the shortest time!" said
Burl.

"Right!" snapped Haines. "The Sun-tap must have been built by a
relatively small team, which probably came in a single explorer ship.
The ship was equipped with automatic factory machinery that could turn
out an adequate base for an uninhabited planet, an airless moon, and so
on--but they didn't have the stuff for a fortified base--and they didn't
have the manpower to build it."

"Another indication of that is the thirty-year delay," added Ferrati.
"Obviously, they arrived in this solar system from somewhere outside it.
We figure that way because otherwise they would have been prepared to do
the job on all the planets in the same trip and start operations at
once. They must have made some observations of this solar system from a
point in space a