The Runit Dome (aka Cactus Dome) is a 377 foot X 18 inch dome of radioactive debris, entombed in concrete, that stems from nuclear tests conducted in the Enewetak Atoll of the Marshall Islands by the United States between 1946 and 1958.

From 1977 to 1980, loose waste and topsoil debris scraped off from six different islands in the Enewetak Atoll was transported here, mixed with concrete, and stored in the nuclear blast crater of the May 1958 Cactus test.