Our planet hides many secrets, both in the depths of the land and the oceans, but they are even more in the Earth's crust.

From time to time, scientists discover mysterious tunnels that lead to the Earth's crust and stretch tens of kilometers into the distance. According to some versions, some of them can connect the continents. As per Planet Today, this assumption seems absurd only at first glance. Moreover, during the construction of a number of such underground structures, technologies were used which are difficult to implement even today.

Who built them? For what purpose? Do they originate from a mysterious powerful ancient Atlantean civilization? These questions are waiting to be answered.

One of the most famous monuments left by ancient genius builders is the city of Petra in the southwest of Jordan, which is completely carved out of rocks. Petra is also called the "pink city" because at sunrise the rocks turn really bright pink. The peak of Petra's development was about 2,000 years ago, when about 20,000 people lived there. The inhabitants left the extraordinary city in the 7th century AD, and the Europeans rediscovered it only at the beginning of the 19th century. Excavations are still going on there, and scientists are sure that even half of the settlement hidden under the ground has not been excavated yet.

Australia's Coober Pedy town is a former settlement of rainbow stone miners. The strangest city in the world. It had to be built underground because the climate there is very harsh. Not even a tree grows. It is very hot there in the summer, and the dingo dogs constantly attacked the miners, so they had to build dugout houses. The underground atmosphere there is quite comfortable, there is no excruciating heat. And the residents come out of the dugout houses at night. Tourists are offered to rent an underground apartment and thus experience the exotic nature of such an existence.

The history of the attempt to build a tunnel to connect the mainland of Russia with Sakhalin Island through the Tatar Strait is also interesting. They started building the tunnel in the 1950s, but left it unfinished. In the early 1990s, the seal of secrecy was removed from the project. And then it became clear that in the 1950s, they were not talking about the construction of a new tunnel, but about the reconstruction of the existing one. No one knows who built the tunnel and when, but some mechanisms and fossils of unknown animals were found inside. All artifacts later disappeared.