Fun fact: Charles bridge (depicted in the video) was damaged during a flood in 1890. With the water level risen, broken down rafts were beating its arches and two of them fell apart, along with three pillars that were undermined by the water. During the reconstruction of the pillars, they decided to make them hollow (to cut on the weight) and you can actually go inside! It's not open to the public but there's a video that shows the inside of a pillar number 6 [0].
[0] https://www.novinky.cz/domaci/clanek/jak-vypada-karluv-most-...