Japan on Monday marked 13 years since a massive earthquake and tsunami hit the country’s northern coasts. Nearly 20,000 people died, whole towns were wiped out and the Fukushima Daiichi nuclear power plant was destroyed, creating deep fears of radiation that linger today.

The contents of the three reactors is still largely a mystery. Little is known, for instance, about the melted fuel’s condition or exactly where it’s located in the reactors. Not even a spoonful of the fuel has been removed yet.

About 880 tons of melted nuclear fuel remain inside the three damaged reactors, and Japanese officials say removing it would take 30-40 years. Experts call that timeline overly optimistic. The amount of melted fuel is 10 times that removed from Three Mile Island following its 1979 partial core melt.