It is used in the late 70s as a burial pit to encapsulating an estimated 73,000 cubic meters (95,000 cu yd) of radioactive debris and soil (including some plutonium-239), scraped from the various contaminated islands. 4,000 US servicemen were involved in the cleanup and it took three years to complete. The dome was built to cover the material.
A report of the US Department of Energy in 2013 found that radioactive materials were leeching out, threatening the already tenuous existence of Enewetak locals.
Read more here in an article of ABC News.
Photo by US Defense Special Weapons Agency.