A glass-walled building protects this icon of Philadelphia history from the elements. You can peek from outside, or join the line to file past, reading about the history and significance of the 2080lb object along the way. The line – and it can be a long one in peak summer months – starts on the building's northern end.

　　The gist of the story: originally called the State House Bell, it was made in 1751, to commemorate the 50th anniversary of Pennsylvania's constitution. Mounted in Independence Hall, it tolled on the first public reading of the Declaration of Independence. The crack developed in the 19th century, and the bell was retired in 1846.