Calamities: The 7 Worst Man-Made Disasters in History

A flawed nuclear reactor design, poorly trained personnel and a stroke of very bad luck: These were the ingredients that made the 1986 Chernobyl disaster the worst nuclear accident of the 20th century. The enormous plume of radiation that escaped from the nuclear power plant contaminated over 38,000 square miles -- an area larger than Indiana -- and exposed some five million people to high levels of radiation. Because radiation-caused cancer takes decades to develop, an estimated 4,000 to 9,000 deaths could eventually result from Chernobyl. Ironically, the now-abandoned area has reverted back to forest, and the site is home to a thriving population of wolves, deer, raccoon dogs, foxes and other wildlife.

Bhopal: For some, the word is synonymous with an environmental disaster of unimaginable horror. But outside India, the events that took place in 1984 are now largely forgotten. A Union Carbide pesticide plant in Bhopal -- a city of roughly one million people -- experienced a malfunction, and poorly trained plant personnel were unable to stop a leak of some 40 tons of deadly pesticide gas. Up to 500,000 people were exposed to the deadly gas cloud, and total deaths from the disaster are somewhere in the range of 8,000 to 10,000. The Bhopal disaster also killed untold thousands of animals, trees and plants throughout the area lost all their leaves, and food and water were contaminated and therefore unfit for consumption. Union Carbide continues to deny responsibility for the catastrophe.

The area near Seveso, Italy, saw one of Europe's worst environmental disasters in 1976. A chemical plant going through routine shutdown procedures accidentally leaked about six metric tons of toxic gas into a largely residential area. The gas cloud that drifted over the Seveso area contained an estimated one kilogram of TCDD, a form of cancer-causing dioxin that also causes severe skin problems including chloracne. Within a few hours, over 37,000 people were exposed to unprecedented levels of dioxin. In the bungled response, several days passed before it was announced that dioxin had been released from the facility; evacuation of the worst-affected areas took several more days.

Could the horror known as the Fukushima disaster have been prevented? According to hundreds of international experts, the answer is a troubling "yes." The nuclear power plant -- one of the world's largest and most powerful -- relied on emergency generators that were dangerously close to a seashore that had experienced several large tsunamis in the recent past. And on March 11, 2011, a tsunami caused by a massive 9.0 earthquake slammed into the coast of Japan, killing over 19,000 and crippling the Fukushima power plant. In the tense hours and days that followed, three of the plant's six nuclear reactors were in full meltdown, spreading radiation throughout a huge area of Japan and, eventually, around the world. The Fukushima accident is the 21st century's worst nuclear disaster -- so far.

Even larger than 1989's Exxon Valdez oil spill, the Deepwater Horizon oil spill in the Gulf of Mexico was the result of cost-cutting measures by oil company BP, failed safety systems and lax government oversight. The spill began on April 20, 2010, when a deadly explosion killed 11 people on a deep-sea drilling platform named Deepwater Horizon. The blast caused a wellhead to rupture, pumping an estimated 5 million barrels of oil into the Gulf. Reports as recent as March of 2012 indicate that the well may still be leaking, despite repeated efforts to cap it. The spill decimated the Gulf Coast's important fishing and tourism industries, fouled beaches and wetlands across hundreds of miles in several states, and killed countless fish, mammals, birds and other animals. The worst environmental disaster in U.S. history isn't over yet, as oil continues to stain beaches in the area, and scientists report ongoing problems like mutations in wildlife populations and the appearance of deadly bacteria throughout the Gulf region.

While cities like Los Angeles and Mexico City may be more renowned for their unhealthy air quality, the worst air pollution event in recorded history took place in London in December of 1952. The city's residents had long relied on coal as a source of heat, and when a cold snap occurred during the first week of December, Londoners stoked up their coal fires. But a weather condition known as a inversion -- where cold, heavy air is capped under a layer of warmer air -- trapped all the region's smoke and fog, creating a near-blackout of poisonous air. Visibility was reduced to one foot (people reportedly couldn't see past their knees to their feet), and the resulting lack of sunlight prompted shivering residents to heap even more coal into their fires. Soon a deadly mix of stinking, sulfurous air clamped down on London for 4 hellish days, indoors as well as outside. The city's hospitals were overwhelmed with the sick and dying, especially young children, the elderly and those with existing respiratory problems. The Big Smoke, a.k.a. the London Smog of 1952, is blamed for up to 12,000 deaths -- mostly from hypoxia, or lack of oxygen -- and forever changed the city's view of it's oft-foggy weather.

Prince William Sound, off the shore of Alaska, was home to one of North America's richest fisheries, and the entire area teemed with wildlife including sea otters, harbor seals, bald eagles, porpoises and several species of whale. That wealth of biodiversity was decimated on the night of March 24, 1989, when the Exxon Valdez oil tanker ran aground in the area, spilling at least 11 million gallons of crude oil. As a black stain spread across some 1,300 miles of pristine coastline, up to a million sea birds, fish, mammals and other wildlife died, despite the valiant efforts of roughly 11,000 clean-up volunteers and workers. The Exxon Valdez spill continues to affect wildlife in the area, since oil was never completely cleaned up from remote, rocky beaches and wetlands. As a result of the disaster, the Oil Pollution Act of 1990 required oil shipping operators to adhere to strict new regulations for environmental health and safety.