After a fire destroyed much of the Palace of Westminster–the headquarters of the British Parliament–in October 1834, a standout feature of the design for the new palace was a large clock atop a tower. The royal astronomer, Sir George Airy, wanted the clock to have pinpoint accuracy, including twice-a-day checks with the Royal Greenwich Observatory. While many clockmakers dismissed this goal as impossible, Airy counted on the help of Edmund Beckett Denison, a formidable barrister known for his expertise in horology, or the science of measuring time.

Denison’s design, built by the company E.J. Dent & Co., was completed in 1854; five years later, St. Stephen’s Tower itself was finished. Weighing in at more than 13 tons, its massive bell was dragged to the tower through the streets of London by a team of 16 horses, to the cheers of onlookers. Once it was installed, Big Ben struck its first chimes on May 31, 1859. Just two months later, however, the heavy striker designed by Denison cracked the bell. Three more years passed before a lighter hammer was added and the clock went into service again. The bell was rotated so that the hammer would strike another surface, but the crack was never repaired.

The name “Big Ben” originally just applied to the bell but later came to refer to the clock itself. Two main stories exist about how Big Ben got its name. Many claim it was named after the famously long-winded Sir Benjamin Hall, the London commissioner of works at the time it was built. Another famous story argues that the bell was named for the popular heavyweight boxer Benjamin Caunt, because it was the largest of its kind.

Even after an incendiary bomb destroyed the chamber of the House of Commons during the Second World War, St. Stephen’s Tower survived, and Big Ben continued to function. Its famously accurate timekeeping is regulated by a stack of coins placed on the clock’s huge pendulum, ensuring a steady movement of the clock hands at all times. At night, all four of the clock’s faces, each one 23 feet across, are illuminated. A light above Big Ben is also lit to let the public know when Parliament is in session.

On April 26, 1986, the world’s worst nuclear power plant accident occurs at the Chernobyl nuclear power station in the Soviet Union. Thirty-two people died and dozens more suffered radiation burns in the opening days of the crisis, but only after Swedish authorities reported the fallout did Soviet authorities reluctantly admit that an accident had occurred.

The Chernobyl station was situated at the settlement of Pripyat, about 65 miles north of Kiev in the Ukraine. Built in the late 1970s on the banks of the Pripyat River, Chernobyl had four reactors, each capable of producing 1,000 megawatts of electric power. On the evening of April 25, 1986, a group of engineers began an electrical-engineering experiment on the Number 4 reactor. The engineers, who had little knowledge of reactor physics, wanted to see if the reactor’s turbine could run emergency water pumps on inertial power.

As part of their poorly designed experiment, the engineers disconnected the reactor’s emergency safety systems and its power-regulating system. Next, they compounded this recklessness with a series of mistakes: They ran the reactor at a power level so low that the reaction became unstable, and then removed too many of the reactor’s control rods in an attempt to power it up again. The reactor’s output rose to more than 200 megawatts but was proving increasingly difficult to control. Nevertheless, at 1:23 a.m. on April 26, the engineers continued with their experiment and shut down the turbine engine to see if its inertial spinning would power the reactor’s water pumps. In fact, it did not adequately power the water pumps, and without cooling water the power level in the reactor surged.

To prevent meltdown, the operators reinserted all the 200-some control rods into the reactor at once. The control rods were meant to reduce the reaction but had a design flaw: graphite tips. So, before the control rod’s five meters of absorbent material could penetrate the core, 200 graphite tips simultaneously entered, thus facilitating the reaction and causing an explosion that blew off the heavy steel and concrete lid of the reactor. It was not a nuclear explosion, as nuclear power plants are incapable of producing such a reaction, but was chemical, driven by the ignition of gases and steam that were generated by the runaway reaction. In the explosion and ensuing fire, more than 50 tons of radioactive material were released into the atmosphere, where it was carried by air currents.

On April 27, Soviet authorities began an evacuation of the 30,000 inhabitants of Pripyat. A cover-up was attempted, but on April 28 Swedish radiation monitoring stations, more than 800 miles to the northwest of Chernobyl, reported radiation levels 40 percent higher than normal. Later that day, the Soviet news agency acknowledged that a major nuclear accident had occurred at Chernobyl.

In the opening days of the crisis, 32 people died at Chernobyl and dozens more suffered radiation burns. The radiation that escaped into the atmosphere, which was several times that produced by the atomic bombs dropped on Hiroshima and Nagasaki, was spread by the wind over Northern and Eastern Europe, contaminating millions of acres of forest and farmland. An estimated 5,000 Soviet citizens eventually died from cancer and other radiation-induced illnesses caused by their exposure to the Chernobyl radiation, and millions more had their health adversely affected. In 2000, the last working reactors at Chernobyl were shut down and the plant was officially closed.