Workers were photographed cleaning the clock faces outside of London’s iconic Big Ben landmark atop the Elizabeth Tower on Monday.

Big Ben's Clocks Cleaned

The clock’s cleaners headed up the Elizabeth Tower Monday to perform routine upkeep on the London fixture. The four men abseiled from windows above the clock and were secured in harnesses for the risky maintenance job. While they work, Big Ben will continue to chime but the clock will hold still at 12 o’clock, reported the BBC.

The abseilers will tackle the four faces of he clock one day at a time. Since the glass is very thin, the cleaners have to tend to the 312 individual pieces of opal glass slowly and carefully. While the clock face is cleaned and the clock is frozen, clock smiths work on its internal mechanics.

"It's one of the pinnacles for any abseiler to be cleaning such a famous structure," abseiler Paul Harrison told the Telegraph. "It's one of the most prestigious structures in the world… we're very proud to be working on it."

The Big Ben clock has been telling time in Westminster, London since 1859. Originally, the tower that houses the Big Ben bell and clock was simply known as Clock Tower, but was renamed in honor of Queen Elizabeth at her Jubilee in 2012. It is the largest four-faced chiming clock tower in the world.