Since the Chernobyl nuclear power plant exploded in 1986, the area of the disaster has been one of the most hazardous places in the world. Thirty years later, the plants, buildings, and even the puppies inside the exclusionary zone are still radioactive. But while the radioactivity in many parts no longer reaches lethal levels, the center of the site is still volatile.

In their new video, RealLifeLore explains why the basement beneath Chernobyl's Reactor 4 is one place you never want to enter. It contains something called the "Elephant's Foot": a dense mass of sand, glass, concrete, control rods, and nuclear fuel that weighs 100 tons despite being less than 7 feet wide. As a result of the disaster, the nuclear plant's core was hot enough to melt through the bottom of its containment vessel, through the floor of the building, and into the basement where it remains today. The mass was so radioactive immediately following the catastrophe that being exposed to it for 300 seconds was enough to kill you within two days. But that didn't stop some Soviet workers from entering the room to photograph it.

Today the Elephant’s Foot is only slightly less deadly: You'd need to spend a whole hour in the room with it to die shortly after, as opposed to just a few minutes. But of all the rooms on Earth you can enter and exit with your body seemingly intact, the Chernobyl basement may be the most dangerous.

[h/t RealLifeLore]