When a nuclear reactor melts down, the fuel rods do not simply melt and stop. Uranium oxide melts at around 2,800 degrees Celsius. Once the structural materials around it - zirconium cladding, steel, concrete, sand - reach those temperatures too, the whole stew flows together into a substance the industry calls corium. At Chernobyl, on the night of April 26, 1986, after the reactor exploded and the graphite caught fire, corium pooled in the room directly beneath the reactor (room 305/2, on the fourth floor of Unit 4) and then began to flow. It poured down through pipes and ducts in what investigators later named the Horizontal Flow. Where one of those flows finally stopped and froze, it left a glistening, wrinkled lump shaped vaguely like the foot of a large animal. The lump is roughly 10 percent uranium by mass.

After the broken dosimeter on the staircase, months passed. In November and December 1986, another dosimetrist named Vasya Koryagin became fixated on the room beneath the reactor itself. The question was whether the steel cross holding up the reactor's lower biological shield was still intact, or whether a hole had opened in the floor of the core. Authorized by his supervisor Vladimir Azmolov, Koryagin entered the freshly built sarcophagus through the Vent Block on the east side of Unit 4. By that time, concrete had been pumped into the corridors of the building to stabilize the structure - some of it intentional, some of it overflow. Koryagin and the team that followed him were the first humans to lay eyes on the Elephant's Foot. The radiation field at the surface of the mass at that point was on the order of 8,000 to 10,000 roentgens per hour - a dose lethal within minutes.

In 1996, ten years after the accident, Artur Korneyev - then deputy director of the New Safe Confinement Project - went into the room with a film camera and a flashlight. The radiation had decreased significantly: the surface was emitting roughly 1,000 roentgens per hour, still extreme but survivable for a brief visit. Korneyev set up an automatic camera with a long exposure, used the flashlight to illuminate the Foot, and walked into his own frame. The result is the most famous photograph ever taken of nuclear meltdown debris - a ghostly figure standing beside a pillar of frozen molten ruin, the film visibly fogged by the gamma radiation pouring through the camera body. Korneyev survived the trip. He continued working at Chernobyl for years afterward and the photograph he made has become the definitive image of what corium actually looks like once it has cooled.

Radiation falls off with time as short-lived isotopes decay. By 1989 the surface dose at the Elephant's Foot had dropped to roughly 1,100 roentgens per hour. By 2016, according to measurements by Sergei Koshilev, it was down to about 100 roentgens per hour. Some researchers have theorized that the rapid early decline was driven by ruthenium-106, whose half-life of about a year matches the curve. The remaining external hazard now comes mostly from gamma-emitting fission products, principally caesium-137, which decays much more slowly. Inside the mass, the chemistry continues to evolve. Zircon crystals are slowly forming as the corium cools, and crystalline uranium dioxide dendrites grow and break down repeatedly. By 2021, scientists described the bulk of the Foot as having a consistency similar to sand - an alpha-emitting dust, dangerous if inhaled or ingested even though alpha radiation cannot penetrate intact skin.

The Elephant's Foot is often called the most radioactive object in history. It is not really - any number of intact spent fuel assemblies in a reactor pool would emit comparable or higher fields - but the comparison gets at why the object captivates people. It is what a meltdown looks like. It is the molten core of a working power station, frozen in the basement, with the human geometry of pipes and concrete still vaguely visible around it. The New Safe Confinement, the giant arch slid over Reactor 4 in 2016, has sealed the whole shelter from the weather and is designed to allow eventual disassembly. The Foot is part of what would have to be dismantled. Until then it sits in the dark, slowly cooling, slowly decaying, slowly turning to a uranium-bearing dust that will outlast the building above it by tens of thousands of years.