A nuclear meltdown is the worst case scenario for a nuclear power plant, causing widespread releases of deadly radiation into the environment that can spread hundreds of miles away impacting wildlife as well as humans. Radiation can not be seen, smelled, or tasted and exposure can lead to serious side effects such as hair loss, skin blisters, tumors, and cancer and if the dose is large enough, death. Since 1952 there have been fourteen meltdowns of varying severity at both commercial, military, and experimental reactors. The top three most disastrous meltdowns include Three Mile Island in 1979, Chernobyl in 1986, and Fukushima Daiichi in 2011. It is important to remember that a nuclear meltdown isn't a singular tragic event as is often the case with natural disasters. The effects of a meltdown can last for hundreds of years as the radiation slowly decays. 

What actually causes a meltdown? A meltdown occurs in a reactor when the fuel isn't being adequately cooled. The fuel rods are kept submerged underwater, with the water acing as a coolant. If the heat generated from the fission reaction is so great that it causes a majority of the water to boil off exposing the fuel rods to air. The Uranium pellets in the rod quickly become so hot that they reach their melting point, burning through the zirconium fuel rod casing and the containment chamber floor causing a widespread release of radiation. 

Source: Associated Press

China Syndrome is a hypothetical disaster that could occur under meltdown conditions. Made popular by the 1979 movie of the same name. The idea states that under the proper meltdown conditions, the nuclear core could melt through the containment structure, and eventually through the crust and core of the earth until it reached China. As the movie states and what is not theoretical is that as soon as the core "hits ground water, it blasts into the atmosphere and sends out clouds of radioactivity. The number of people killed would depend on which way the wind was blowing, rendering an area the size of Pennsylvania permanently uninhabitable." Just 12 days after the movie was released the meltdown at Three Mile Island occurred. The fuel did not manage to melt through the bottom of the reactor pressure vessel, however due to concerns of hydrogen build-up, the plant operators vented radioactive gasses into the atmosphere. The nuclear disaster at Fukushima Daiichi was a much more severe meltdown though, to find out if that possible could have been a China Syndrome check out our video here