What mental picture comes to mind when you think of London, England? If you're like many people, you probably think of the iconic Clock Tower at the Palace of Westminster. Featured in many movies and pictures, the Clock Tower has become a well-known symbol of England and London.

One of the world's most famous tourist attractions, the Clock Tower features four huge clock faces. At one time, it was the largest four-faced clock in the world, an honor now claimed by the Allen-Bradley Clock Tower in Milwaukee, Wisconsin. The Clock Tower also houses bells that chime every quarter hour, with the largest bell — called the "Great Bell" — chiming every hour on the hour.

Many Londoners refer to the Clock Tower, the clocks and the Great Bell, collectively, as “Big Ben." However, Big Ben is actually the nickname of only the Great Bell.

How did the Great Bell get the nickname Big Ben? There's some debate about that. Some people believe it was named after a boxer named Benjamin Caunt, who was the English Heavyweight Champion.

Others believe the Great Bell was named after Sir Benjamin Hall, who oversaw the installation of the Great Bell. Since Sir Benjamin Hall's name is inscribed on the Great Bell, many people believe it's most likely that he was the source of the Great Bell's nickname.

The Great Bell earns its Big Ben nickname. At over 7 feet tall and over 9 feet wide, the Great Bell weighs an astounding 13.5 tons!

It first chimed in July 1859. Just two months later, though, it cracked. To repair the bell, a square piece of metal was chipped from the rim around the crack.

The bell was then turned an eighth of a turn, so that it would no longer be struck where it was cracked. The cracked bell has hung in the Clock Tower and chimed with a unique twang ever since.

Completed on April 10, 1858, the Clock Tower stands 316 feet high (about 16 stories). The Clock Tower's structure consists of brick, limestone and a framed spire of cast iron.

It houses four clocks designed by Augustus Pugin. The clocks' dials are set in an iron frame 23 feet in diameter.

The dials are composed of 312 pieces of opal glass, similar to a stained-glass window. Each clock dial features the following Latin phrase — DOMINE SALVAM FAC REGINAM NOSTRAM VICTORIAM PRIMAM — which means “O Lord, keep safe our Queen Victoria the First."

Londoners look to Big Ben on New Year's Eve, as the familiar chimes mark the start of each new year. The chimes also mark the 11th hour of the 11th day of the 11th month every year on England's Remembrance Day, which marks the day and time that World War I ended in 1918.

The Clock Tower's clocks are known for their reliability. Only during times of war, such as during World War I and World War II, have the chimes been silenced and the clock faces darkened to help prevent enemy attacks.