Forty years removed from the Chernobyl Nuclear Power Plant disaster in 1986 that saw the catastrophic failure of reactor 4, Chernobyl itself remains a tragic time capsule, serving as a sobering reminder to the dangers of nuclear energy –- and the consequences of human error in the absence of nuclear safety. Chernobyl still bears the nuclear scars from four decades ago; some are fading, while others are raw as ever. Underneath the surface of Chernobyl, one such scar is “The Elephant’s Foot,” what might be one of the most dangerous man-made objects ever produced, despite its unintentional creation.

Beneath the ruins of reactor 4 lies the resting place of The Elephant’s Foot: a black, lava-like mass of corium, given the name for its uncanny resemblance to the foot of a massive elephant. Corium, also known as a lava-like fuel-containing material (LFCM), is the result of a molten mixture of nuclear fuel and reactor materials. In the case of The Elephant’s Foot, it is also composed of several other elements and materials that it combined with as it melted through them, descending into a maintenance corridor beneath the reactor, where it remains a radioactive slag heap.

The Elephant Foot of Chernobyl was discovered several months after the initial accident in December 1986, when dosimetrists stumbled upon it while investigating the corridors beneath the reactor. As a direct result of the core meltdown, the core, fuel rods, graphite moderator components, and nuclear fuels all began to melt into one another. This formed a radioactive lava-like sludge that continued to melt through the bottom of the reactor structure, changing composition as it interacted and melted with other materials such as steel, glass, sand, and concrete as it melted through floors. As it came to rest and started cooling in the basement of the building, its appearance looked more like a black ceramic, and its final composition included multiple fission products, molten building materials, and elements like uranium and zirconium.

This toxic amalgamation of nuclear fuel and fission byproducts is what forms corium, a uniquely human made danger, born out of nuclear disaster and human error. Corium has only been created five times in human history: once at Chernobyl, once at the Three Mile Island accident, and three times during the Fukushima Daiichi plant accident. Upon its discovery, the corium mass that makes up The Elephant’s Foot emitted roughly 10,000 roentgens per hour, and just a few minutes of exposure would be lethal. The Elephant’s Foot is just a small part of the estimated 100 tons of Corium beneath Chernobyl. Over the years, the danger posed by the Elephant’s Foot has decreased as its radioactive materials have decayed. Though, it’s far from safe, and will likely pose radiation concerns for decades.

As the corium masses throughout Chernobyl cooled, a unique crystallized mineral began to form on them, known as Chernobylite. These crystal formations occur after the corium is exposed to air and steam, and they are found nowhere else on earth. The closest mineral is Trinitite, soil that was fused into a radioactive glass-like material after the nuclear explosion test in 1945, known as the Trinity Test. Chernobylite crystals are another distinct danger created from the nuclear fallout, as they contain high amounts of uranium and zirconium, in addition to other contaminated nuclear products.

Studying corium and Chernobylite have been a challenge, as their very creation is exclusive to nuclear disaster. And that says nothing of the extreme radiation danger that real world samples represent. Although, over time, scientists have been able to successfully simulate certain lava-like fuel-containing materials (LFCMs) in order to better understand their behavior. A study published by Nature documents the recreation of these LFCMs in order to study how they corrode, which is important for managing sites like Chernobyl in the long term. The same study was also able to successfully synthesize a material similar to Chernobylite, which may provide more clarity into the process of how the mineral is created during corium formation, which may in part help shape a safer future for nuclear reactors.