The summit of Olympus Mons had for the most part been left alone. Oh there were a few low boulder hermitages on rim overlooks, and a piste had been built on the northeast lava flow that broke the escarpment ring surrounding the volcano, for easy access to the festival complex at Crater Zp; but other than that, there was nothing to show what had happened to the rest of Mars, which from the rim of the caldera was entirely invisible, under the horizon of the encircling escarpment. From its rim Olympus Mons appeared to be the world entire. The local Reds had decided against putting a protective molecular dome over the caldera, something they had done over Arsia Mons; so no doubt there were bacteria, and perhaps some lichens that had blown over on winds and floated down into the caldera and survived; but at pressures little higher than the original ten millibars, they were not going to flourish. Probably the survivors were mostly endochasmoliths, so there would be no sign of them. It was a lucky thing for the Red project that Mars’s stupendous vertical scale kept air pressures so low on the big volcanoes; a free and effective sterilization technique.

Sax took the train up to Zp, and then a car on up to the rim, a taxi van driven by the Reds who controlled access into the caldera. The car came to the edge of the rim, and Sax looked down.

The caldera was multiringed, and big: ninety kilometers by sixty, about the same size as Luxembourg, Sax recalled hearing. The main central circle, by far the largest, was marred by overlapping smaller circles to the northeast, center, and south. The southernmost circle cut in half a slightly older, higher circle to the southeast; the meeting of these three arcuate walls was considered one of the finest climbing areas on the planet, Sax was told, with the greatest height anywhere in the caldera, a drop from 26 kilometers above the datum (they used the old term rather than sea level) down to 22.5 kilometers on the southernmost crater floor. A ten-thousand-foot cliff, the young Coloradoan in Sax mused.