Chernobyl, specifically the basement beneath Reactor 4
“The so called Elephant’s Foot is a solid mass made of melted nuclear fuel mixed with lots and lots of concrete, sand, and core sealing material that the fuel had melted through. It is located in a basement area under the original location of the core. In 1986 the radiation level on the ”Elephant’s Foot” was measured at 10,000 roentgens per hour, and anyone who approached would have received a fatal dose in under a minute. After just 30 seconds of exposure, dizziness and fatigue will find you a week later. Two minutes of exposure and the body cells will soon begin to hemorrhage; four minutes: vomiting, diarrhea, and fever. At 300 seconds you have two days to live.”
The radiation is so strong that it even affects the film and makes photos of it appear grainy.
They even had to build a giant concrete sarcophagus to close off the radiation from the outside world.
Oh I also forgot to mention, the contents will remain radioactive for at least 100,000 years.