20. Pravcicka Brana, Czech Republic

Until 1982, Pravcicka Brana was the most well-known attraction in the Czech Republic. After that, it was forbidden for tourists to visit it. Unless you have a time machine, then you are out of luck to climb over this archway, but you can still view it from afar. 

The government closed Pravcicka Brana because tourists were accelerating corrosion, which would cause it to collapse. The archway will eventually collapse one day due to natural corrosion, but the ban slowed down that process.