The Ross Ice Shelf is the largest ice shelf of Antarctica, with an area comparable to that of France's. It is several hundred meters thick. The nearly vertical ice front to the open sea is more than 600 km (370 mi) long, and between 15 and 50 meters (50 and 160 ft) high above the water surface. Ninety percent of the floating ice, however, is below the water surface.
Most of Ross Ice Shelf is in the Ross Dependency claimed by New Zealand.
In 1966, an iceberg broke off the Ross Ice Shelf and began drifting north. Many correctly suspected that the iceberg had something to do with the Mountains of Madness or the Old Ones, as the iceberg drifted north (even crossing the Equator without melting) until Spring, 1968, when it reached the San Francisco coast.
The iceberg landed near the San Francisco Zoo, and several shoggoths disembarked and attacked various sites in the city containing relics of the Old Ones.