In April of 1986, Unit 4 of the Chernobyl nuclear reactor in Ukraine (then part of the USSR) blew up, causing a meltdown. The rods of nuclear fuel turned to liquid and melted their containers, producing a highly radioactive substance called corium. Corium flowed through the building, and in a week's time, eleven tons of it settled on the floor in a shape that was later dubbed the "elephant's foot." The corium was so toxic it would kill anyone in the room within minutes, but this picture was taken ten years afterward. Oh, it was still radioactive, and it is still decaying today, 36 years later. The worker on the left in the picture is Artur Korneyev, a Kazakhstani nuclear inspector who still lives in Slavutich, Ukraine. The photograph, taken in 1996, was tucked into a collection of images given to the US in the late '90s without context.
It took years to determine who the person in the photo is, who took it, and what was going on, but you can now read about Artur Korneyev and the elephant's foot at Atlas Obscura.