The Elephant's Foot is the nickname given to a large mass of corium, composed of materials formed from molten concrete, sand, steel, uranium, and zirconium. The mass formed beneath Reactor 4 of the Chernobyl Nuclear Power Plant, near Pripyat, Ukraine, during the Chernobyl disaster of 26 April 1986, and is noted for its extreme radioactivity. It is named for its wrinkled appearance and large size, evocative of the foot of an elephant.

Discovered in December 1986, the “foot” is located in a maintenance corridor below the remains of Reactor No. 4, though the often-photographed formation is only a small portion of several larger corium masses. It has a popular reputation as one of the most radioactive objects in history, though the danger has decreased over time due to the decay of its radioactive components.

The Elephant's Foot is a mass of black corium with many layers, resembling tree bark and glass. It was formed during the Chernobyl disaster of April 1986 from a lava-like mixture of molten core material that had escaped the reactor enclosure, materials from the reactor itself, and structural components of the plant such as concrete and metal. The Foot was discovered in December 1986 in Room 217/2, 15 meters to the southeast of the ruined reactor and 6 meters above ground level. The material making up the Elephant's Foot had burned through at least 2m of reinforced concrete, then flowed through pipes and fissures and down a hallway to reach its current location.

The Elephant's Foot is a black ceramic composed primarily of silicon dioxide, with smaller amounts of other oxides, primarily uranium, calcium, iron, zirconium, aluminum, magnesium, and potassium. Over time, zircon crystals have started to form slowly within the mass as it cools, and crystalline uranium dioxide dendrites are growing quickly and breaking down repeatedly. Despite the distribution of uranium-bearing particles not being uniform, the radioactivity of the mass is evenly distributed. The mass was quite dense and unyielding to efforts to collect samples for analysis via a drill mounted on a remote-controlled trolley, and armor-piercing rounds fired from an AK-47 assault rifle were necessary to break off usable chunks. By June 1998, the outer layers had started turning to dust and the mass had started to crack, as the radioactive components were starting to disintegrate to a point where the structural integrity of the glass was failing. In 2021, the mass was described as having a consistency similar to sand.

At the time of its discovery, about eight months after formation, radioactivity near the Elephant's Foot was approximately 8,000 to 10,000 roentgens, or 80 to 100 grays per hour, delivering a 50/50 lethal dose of radiation (4.5 grays) within 3 minutes. Since that time, the radiation intensity has declined significantly, and in 1996, the Elephant's Foot was briefly visited by the deputy director of the New Safe Confinement Project, Artur Korneyev, who took photographs using an automatic camera and a flashlight to illuminate the otherwise dark room.

The Elephant's Foot is roughly 10% uranium by mass, which is an alpha emitter. While alpha radiation is ordinarily unable to penetrate the skin, it is the most damaging form of radiation when radioactive particles are inhaled or ingested, which has renewed concerns as samples of material from the meltdown (including the Elephant's Foot) turn to dust. Nevertheless, the corium still poses an external gamma radiation hazard due to the presence of fission products, mainly caesium-137.

• Chernobylite
• Trinitite

51°23′21″N 30°05′54″E / 51.3892°N 30.09833°E / 51.3892; 30.09833