If you've ever lost the key to your house, you know how much trouble you have to go through to open the door again. But some doors in the world aren't meant to be opened at all because they're hiding some mysteries behind them! The famous Taj Mahal in India, for example, has a lot of doors that are always closed. Experts think that if someone opened these doors after they've been closed for so long, it'd release carbon monoxide, which could lead to the appearance of cracks in the minarets of the Taj Mahal. Find out more about these and other mysterious doors around the world!

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