Olympus Mons

Largest of the ancient volcanoes of Mars, and (according to background in TBONWM) the largest shield volcano on any known planet -- a fact which has apparently turned it into something of a tourist destination. Stopping in its caldera on her first off-planet jaunt in HW, Dairine stumbles across a commemorative monument / beacon left by an alien climbing expedition; and Rhiow and Saash spend a great deal of energy on a timeslide-enabled rescue mission during which they have to retrieve an Andorrin expedition which has become lost in the volcano's caverns at a time in the past just before its climactic eruption. (HW, TBONWM, AWOM)