125 Kilometres Long and Named After a Monster: Japan’s Undersea ‘Godzilla’ Joins a Global Heritage List
It is 125 kilometres long, 55 kilometres wide and sits on the Philippine Sea plate southeast of Okinotorishima, a remote Pacific island administered by Tokyo. On Friday the Japan Coast Guard announced that the Godzilla Megamullion — a vast dome of seafloor named for its size after the famous film monster — has been selected for the International Union of Geological Sciences’ list of 100 Geological Heritage Sites.
The recognition was announced at an IUGS meeting held in Portugal from 4 to 6 November, the third round of a programme that named a first hundred sites in 2022 and a second hundred in 2024. For the first time, the list also took in geologically important places in the ocean. Alongside the Godzilla Megamullion, the new marine entries include the Challenger Deep, the Daiichi-Kashima Seamount, the Boso Triple Junction off Japan’s Boso Peninsula, the Rodrigues Triple Junction and the Galapagos hydrothermal system.
A dome built from the mantle
A megamullion forms when faulting associated with seafloor spreading peels back the crust and exposes mantle rock as a dome on the ocean floor. The Godzilla Megamullion is about ten times larger than a typical example, roughly three times the area of Tokyo Metropolis, and is thought to be the largest of its kind ever found. The Japan Coast Guard discovered it in 2001 during a survey to define the continental shelf.
Researchers named it after Godzilla because of that scale. In 2021 the Sub-Committee on Undersea Feature Names registered the name internationally — the first megamullion in the world to receive that official treatment — and in 2022 it added 14 names for the formation’s individual features. An international research group led by Japanese scientists has used the site to study the structure of the upper mantle and how the Philippine Sea plate formed. The Coast Guard said the IUGS recognised it as a global reference point for that work.
Not Japan’s first entry, but its first from the deep
Japan already has several sites on earlier IUGS lists, among them the Nojima Fault in Hyogo Prefecture, which produced the 1995 Great Hanshin Earthquake, as well as Genbudo Cave in the same prefecture, the Holocene coral-reef terraces of Kikai Island in Kagoshima and Heisei-Shinzan in Nagasaki. The Godzilla Megamullion is different: it cannot be visited on foot. Its new status is a prize for years of mapping, naming and research rather than a new stop on a tourist trail. It is also a reminder that some of the planet’s most useful geology is still out of sight, on the ocean floor, waiting for a survey ship to draw it properly.