On this day in 1965, construction was completed on the St. Louis Arch. Not only is it included on the U.S. National Register of Historic Places, it’s also a pretty impressive illusion: despite appearing taller than it is wide, the arch is precisely 630 feet tall and 630 feet wide. It is, in fact, the world’s largest man-made optical illusion.

Here’s something else rather interesting about the construction of the Gateway Arch: there are 16 small windows on each side of the observation deck at the top. And when we say small, we really mean quite tiny – each window measures only 7″x 27″. The reason for the petite windows? It all comes down to pressure: it took over 500 tons of pressure to jack the north and south legs of the arch apart so that the final top piece could be placed. Windows that were any larger simply couldn’t have handled that kind of pressure.

Speaking of pressure, don’t let it be your arch-enemy! If you’d placed an ad with us and you’d like to change the wording, add more information or lower the price, we’re happy to help you do that. Just use “Change or Correct Ad” on or give us a call at 1-800-537-9377.