Huge nuclear weapon tests were done in the areas around the Marshall Islands. After the world collectively decided that nuclear weapons weren't the best way to enforce their own varying interpretations of "freedom," there was a lot of radioactive cleanup to be done.
Enter Runit Island, part of what is known as the Eniwetok Atoll. Over a 10 year period from 1948-1958, the atoll was the site of a half dozen nuclear tests that stretched in magnitude up to 10 megatons. This obviouisly left a lot of the ground contaminated with radiation. Not to mention the tens of thousands of cubic meters of irradiated soil from all the other test sites in the general area. So what was the U.S. government to do to cleanup all this mess? If you said, "build a really creepy-looking dome on top," you are correct. Using Portland cement, the United States mixed in over 100,000 cubic meters of contaminated soil, then buried the whole concoction in an atomic blast crater on the north end, known as Runit Island. We would've fired it into outer space instead of this $250 million cleanup effort, but that would mainly be because it would look cool and scare off alien invaders.