Anvils are a peculiar relic in popular culture. Once widespread, they are now the quintessential example of heavy and dense objects – possibly, Wile E. Coyote is to blame for that. Even though they really are heavy and dense, they can float if you switch water for a denser liquid like mercury.
This is what Youtuber and backyard experimenter Cody Don Reeder has done. He got a large vat, filled it with mercury, and then dropped a 49.9-kilogram (110-pound) iron anvil in it. And the anvil does indeed float. Cody also shows how the sound changes when you hit the floating anvil with a hammer, compared to hitting it in normal conditions outside the mercury bath.
The reason why this happens is that mercury is a lot denser than iron. At room temperature and standard pressure, a cubic centimeter of mercury weighs 13.5 grams (0.48 ounces) – almost double the 7.87 grams (0.28 ounces) per cubic centimeter of iron. Reeder points out that it floats better than wood floats on water. Mercury is so dense that elements such as lead, silver, and copper would also float on it.
Given the fact that mercury is toxic, just a reminder to not try this at home. And if you really want to try some density experiments, there are safer ones and even tastier ones, including making layered cocktails and mocktails. Might not be as visually striking, but definitely need fewer safety precautions.