R'lyeh

The corpse-city built by the Great Old Ones in ages before terrestrial life existed, its architecture deliberately non-Euclidean so that ordinary geometry misleads anyone who walks its angles. It lies drowned and dreaming for all but the rare, unpredictable interval when seismic upheaval lifts it back into the air — and takes it down again just as abruptly.

Whereabouts: The South Pacific, at coordinates the surviving testimony gives only approximately — a section of ocean floor that surfaces and resubmerges with the alignment of the stars.

Access: Reached only once on record, by the chartered yacht Alert, after an undersea earthquake heaved the city's spires above the waterline; the crew that landed there did not plan the visit and could not have repeated it.