The floor of the main caldera was marked by a great number of curving fault patterns, concentric with the caldera walls: arcing ridges and canyons, across which ran some straighter escarpments. These features were all explicable, they had been caused by recurrent caldera collapses following the sideslope drainage of magma from the main chamber under the volcano; but as he looked down from their perch on the rim, it seemed to Sax a mysterious mountain— a world of its own— nothing visible but the vast embayed rim, and the five thousand square kilometers of the caldera. Ring on ring of high curved walls and flat round floors, under a black starry sky. Nowhere were the encircling cliffs less than a thousand meters tall. As a rule they were not completely vertical; their average slope appeared to be just steeper than forty-five degrees. But there were steeper sections all over the place. No doubt the climbers flocked to the very steepest sections, given the nature of their interest. There looked to be some very vertical faces out there, even an overhang or two, as right under them, over the confluence of the three walls.

OLYMPUS MONS CALDERA

• • •

“I’m looking for Ann Clayborne,” Sax said to the drivers, who were rapt with the view. “Do you know where I could find her?”

“You don’t know where she is?” one asked.

“I’ve heard she’s climbing in the Olympus caldera.”

“Does she know you’re looking for her?”

“No. She’s not answering her calls.”

“Does she know you?”

“Oh yes. We’re old— friends.”

“And who are you?”

“Sax Russell.”

They stared at him. One said, “Old friends, eh?”

Her companion elbowed her.

They called the spot they were at Three Walls, sensibly enough. Directly under their car, on a little slump terrace, there was an elevator station. Sax peered at it through binoculars: outer-lock doors, reinforced roofing— it could have been a structure from the early years. The elevator was the only way down into this part of the caldera, if you did not care to rappel.

“Ann resupplies at Marion Station,” the elbower finally said, shocking her codriver. “See it, there? That square dot, where the lava channels from the main floor cut down into South Circle.”

This was on the opposite rim of the southernmost circle, which Sax’s map named “6.” Sax had trouble making out any square dot, even with the binocular’s magnification. But then he saw it— a tiny block just a bit too regular to be natural, although it had been painted the rusty gray of the local basalt. “I see it. How do I get there?”

“Take the elevator down, then walk on over.”

• • •