Containing tonnes of nuclear waste the US left there during Cold War testing and there is a large risk of it leaking due to rising sea levels. Now rising sea levels and climate change threaten to unleash highly radioactive plutonium into the Pacific Ocean.
https://www.news.com.au/technology/environment/marshall-islands-concrete-dome-holding-nuclear-waste-could-leak/news-story/52169e48cdfe041682e9e6c5b103fd6d
https://www.news.com.au/technology/environment/marshall-islands-concrete-dome-holding-nuclear-waste-could-leak/news-story/52169e48cdfe041682e9e6c5b103fd6d