The official name of the huge arch in
St. Louis, Missouri is the
Jefferson National Expansion Memorial. The arch is a
catenary arch; the sides were built separately, and the
keystone put in between last. There's a
cable car inside that can take
you to the top, where you can look out windows at the
surrounding landscape.
It's really a quite
awesome structure.
Underneath the arch (underground) is a museum
containing (among other things) a replica of the Jefferson Memorial, and the entrance to
the cable car ride.