7. Pluto’s Gate, Turkey

While Turkey has many incredible places you can visit, there is one off-limits location for your safety. Pluto's Gate in Hierapolis, Turkey, dates back to ancient times, and people didn't dare to go near it. The legend said that no one could survive there, and a historian threw sparrows inside to test it, and they immediately died. 

It was cruel to use harmless birds, but it lead to an interesting discovery. In 1965, scientists confirmed this legend when they found that the CO₂ levels were so high, and at night it got cold enough so that it formed a lake on the bottom of the gate. Any living being could not survive more than a few seconds there.