Copyright 2016 NPR. To see more, visit http://www.npr.org/.

For only the third time in 157 years, the bell at London's landmark clock will not toll the hours.

Big Ben resides in Elizabeth Tower at the Houses of Parliament. Starting next year, the bell will go silent for a few months as part of a three-year repair plan.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

Pittsburgh has more than 440 bridges in its city – more than Venice, Italy – earning it the nickname the “City of Bridges.” For many residents, the bridges represent the city’s historic ties with industrial production, engineering and steel.

But at least 20 bridges are now labeled “structurally deficient” by state and local officials, often due to age and weather damage. The Liberty Bridge, which once carried an estimated 54,000 vehicles a day, is now undergoing a three-year reconstruction estimated to cost almost $90 million.

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