Atlantis Mystery (l'Énigme de l'Atlantide) by the Belgian artist Edgar P. Jacobs was the seventh comic book in the Blake and Mortimer series, first published in Tintin magazine from March 30, 1955, to May 30, 1956. It appeared in book format in 1957.
Plot
During a speleological expedition deep in the caves of the Azores, Blake and Mortimer are cut off by their old enemy Olrik, trapping them in a gaseous cavern. Surviving the odds, they are captured by a race of technologically advanced men, who claim to be descendants of survivors from Atlantis. Then, immersed in a sinister plot led by Commander Magon to topple the wise Basileus and lead modern Atlantis in conquest of the surface world, they ally themselves with Crown Prince Ikaros to thwart Magon's megalomaniac plans. But the outcome is far from certain.
Professor Philip Mortimer takes his vacation to São Miguel, an island of the Azores, to do a little speleology. While exploring a hole called by locals "O furo do Diabo", he finds a radioactive rock and cannot help making a rapprochement with the orichalcum mentioned by Plato, the mysterious metal of Atlantis, this legendary civilization so-called swallowed up by the waves. He then called to Captain Francis Blake to further exploration of this cave with him. But the two friends quickly discover that they are not the only ones interested in this case. Indeed, a foreign power, having intercepted the letter from Mortimer to the address of Blake, is interested by this potential source of nuclear energy and Olrik hired to retrieve the rock. He disguises himself as a native of the country and managed to accompany Blake and Mortimer in their exploration. Although Olrik is unable to retrieve the orichalcum, he managed to condemn Blake and Mortimer to stay trapped in the cave. Driven out by sulphurous fumes, the two friends have choice to go further in a tunnel, hoping to find another way out. After having escaped death repeatedly, they discover a wealth of orichalcum and eventually vanish, irradiated.
When they wake up, not only are they perfectly healed but, they left the cave, finding himself in a futuristic world. They are taken to the head of the place which is called Basileus. It condemns them to stay until the end of their days in this place. Blake and Mortimer are placed under the protection of a certain prince Icare who explain the situation: they are in Atlantis. Indeed, 12,000 years ago, Atlantis ruled the world from an island in the middle of the Atlantic. But the collision between Earth and a huge celestial body caused the immersion of the continental coasts and island. The few survivors of the Atlantean civilization then decided to build a new and secret Atlantis in the bowels of the Earth. Since then, the Atlanteans, much more evolved than the inhabitants of the surface thanks to the immense energy source that constitutes the orichalcum, watching the surface of the Earth thanks to what earthlings call flying saucers.
So that Blake and Mortimer are starting their new lives in Poseidopolis, capital of Atlantis, they are victims of murder attempts. He then appears to Icarus that Atlantis is threatened with a serious danger. To confirm his suspicions, he went secretly, accompanied by two earthlings, to the great gate that separates the Atlantis of the Kingdom to the barbarians. These are the descendants of wild peoples who lived on Atlantis and Atlantis were greeted in their underground area, now rejected in a territory behind the Great Gate as a result of attempts to revolt.