The gigantic Runit Dome in the Marshall Islands, a concrete coffin holding 3.1 m cubic feet of US-produced nuclear waste. It's now at risk of collapsing, leaking out radioactive substances left behind from some of the world’s largest atomic weapons tests
https://www.latimes.com/projects/marshall-islands-nuclear-testing-sea-level-rise/

Between 1946 -1958, the US conducted 67 nuclear tests in the Marshall Islands. When the tests stopped, they took the waste from the islands, along with contaminated soil from Nevada testing sites, dumped it into a crater left from a nuclear detonation, and covered it in concrete

They then declared the matter closed. Now that the structure is at risk of collapsing, officials in the Marshall Islands have asked the US government for help; they have declined, saying the dome is on their land and therefore the responsibility of the Marshallese government

The story is long, complex and obviously ongoing — no Twitter thread could ever do it justice. You can find out a lot more following the links in the LA Times article; and here's a great link if you'd like to read about the containment structure itself https://marshallislands.llnl.gov/ccc/Hamilton_LLNL-TR-648143_final.pdf