St. Louis' identifying landmark, the Jefferson National Expansion Memorial (aka The Gateway Arch), has been included on the World Monuments Fund's 2014 Watch List for at-risk monuments.

The Arch's issues with corrosion have been known for some time and the Arch is still structurally sound. However, WMF says it's not just corrosion that landed the Arch on their list:

...As is the case with many important examples of modern architecture, the preservation of twentieth-century materials and structural designs has proven challenging. The unusual shape and extreme height of the Arch compound these difficulties, as do economic trends that have led to decreased government funding for the stewardship of national monuments. The complexity of preserving both the structural integrity and aesthetic qualities of the Arch compels cooperation between the public and private sectors to ensure that adequate expertise and resources are brought to bear in the preservation of this important landmark and national symbol.

(UPI/Bill Greenblatt)

In an interview with PBS' News Hour the WMF's Bonnie Burnham discussed the list and what it means for monuments to be included (discussion of the Arch at the 5:00 mark):

The 2014 list includes 5 monuments in the United States: