e constructions from the photos, sir. This had been missed by
the mapping party. It's easy enough to see why when you see the
pictures."

"This the only one?"

"Yes, sir."

"How can you be sure of that, Captain Meford? It's a large planet."

"I had one of the machines scan the remaining maps for geometrical
patterns, sir."

"Isn't that done routinely?" Mr. Tucker asked rather sharply.

"Yes, sir. But you see, we've always expected that if we were ever going
to encounter intelligent life on a planet, it would be rather
widespread. Accordingly--and this is the routine procedure, sir, used,
as far as I know, by all contact parties--we ran through a statistically
significant sample of the terrain. There was nothing on Miracastle out
of the ordinary. There was the typical, low-order vegetable matter,
about what we always find. It was a very typical planet, sir."

The third man from the Earth Committee, Mr. Wallace, seldom spoke. When
he did, his voice was mild, and there was a sense of child-like wonder
in his tone. "The natives?" he asked.

"They ... had fled when we discovered the city."

"Where did they flee to?" Mr. Wallace asked.

* * * * *

Captain Meford glanced upward. Other eyes followed to end just below the
edge of the view screen. Above stood the sheer face of the cliff. Clouds
roiled below the summit, obscuring it from view.

"There is a long sloping plateau up there, and a series of natural caves
back in the next cliff face," Captain Meford said. This did not seem
adequate. He continued: "Most of the air-changing activity starts in the
low-lying areas, at first around the dome positions. It advances along
an elevation front, gradually drifting up. Little tongues are carried up
in advance by the heated currents. The aliens retreated before it. On
the plateau you can see the sentries. I guess they posted themselves
there, at intervals, between the edge and the new caves, to define the
limits of safety. They died there. Six of them. The rest, several
hundred, reached the caves. They are dead, too."

"I see," Mr. Wallace said.

"When you first discovered them--?" Mr. Ryan asked after a moment.

Captain Meford hesitated.

Mr. Tucker said: "I believe one of your men killed himself last
night--wasn't it? A technician? I was told he felt you could reverse the
air-changing equipment in time to save the aliens. I understand that was
very much on his mind for the last wee