This is the lethal "elephant's foot" at Chernobyl

"By the fall of 1986, the emergency crews fighting to contain the nuclear disaster at Chernobyl made it into a steam corridor beneath failed reactor Number 4. Inside this chamber they found black lava that had oozed straight from the core. The most famous formation was a solid flow that their radiation sensors firmly told them not to approach. With cameras pushed in from around a corner, the workers dubbed the dimly lit mass “the Elephant’s Foot.” According to readings taken at the time, the still hot portion of molten core put out enough radiation to give a lethal dose in 300 seconds...

Oozing through pipes and eating through concrete, the radioactive lava flow from reactor Number 4 eventually cooled enough to solidfy. The result was a collection of stalactites and stalagmites, steam valves clogged with hardened lava, and the large black mass that would later be dubbed the Elephant’s Foot...

But despite the dangers, we have pictures of the deadly mass. How? From a safe distance, workers—or “liquidators” as they were called—rigged up a crude wheeled camera contraption and pushed it towards the Elephant’s Foot. Careful examination determined that it wasn’t all nuclear fuel. In fact, the mass was comprised of only a small percentage of fuel; the rest was melted concrete, sand, and core shielding that all melted and flowed together. The material was dubbed “corium,” after the part of the reactor that spawned it. Over time, the Elephant’s Foot decomposed. It puffed dust and its surface cracked..."

More information at Nautilus.