 Philadelphia's history attracts many tourists, with the Independence National Historical Park (which includes the Liberty Bell, Independence Hall, and other historic sites) receiving over 5 million visitors in 2016.The city welcomed 42 million domestic tourists in 2016 who spent $6.8 billion, generating an estimated $11 billion in total economic impact in the city and surrounding four counties of Pennsylvania.Philadelphia International Airport is undergoing a $900 million infrastructural expansion to increase passenger capacity and augment passenger experience; while the Port of Philadelphia, having experienced the highest percentage growth by tonnage loaded in 2017 among major U.S. seaports, was in the process of doubling its capacity in order to accommodate super-sized post-Panamax shipping vessels in 2018.