Beneath the St. Louis Arch is a wonderful underground museum, a key part of the Jefferson National Expansion Memorial run by the National Park Service. It is not well known even to people in St. Louis. Totally below ground beneath the Arch itself, it is as large as a football field. The Museum of Westward Expansion preserves some of the rarest artifacts from the Lewis and Clark Expedition, which started from St. Louis over two centuries ago and began a century of western expansion.
Yes, it is also the way to enter the vertical tram ride up the Arch, but I recommend you skip that ride and spend the time touring the museum. The museum has some fantastic exhibits and the lines for the Arch ride are long. Remember, if you go up in the Arch, you will be disappointed that you cannot any longer see the beautiful Arch (because you are in it)!