The Maracot Deep

The Maracot Deep is an adventurous science-fiction novella in which Arthur Conan Doyle sends readers on an undersea journey to the lost city of Atlantis. Professor Maracot, an intrepid explorer reminiscent of Professor Challenger, leads a small expedition - including a young engineer and a hardy diver - to the depths of the Atlantic Ocean in a specialized diving bell. There, in the lightless abyss, they make an astonishing discovery: an air-filled sub-marine cavern housing survivors of ancient Atlantis, a civilization presumed destroyed millennia ago.
Doyle vividly imagines the Atlanteans: they possess advanced technology (crystal energy devices) and live under the rule of an immortal high priest who has sinister psychic powers. As Maracot and his companions acclimate to this hidden society, they face wonders and perils. Giant deep-sea creatures roam outside the city's dome; inside, the visitors get caught in the power struggle with a malevolent sea-demon the Atlanteans worship.
The climax is high fantasy - Maracot matches wits with this demon (an evil intelligence) in a battle of science vs. dark magic. Ultimately, with the demon banished and Atlantis beginning to crumble, the heroes escape dramatically back to the surface. The Maracot Deep combines Doyle's scientific curiosity with swashbuckling fantasy, offering a Jules Verne-esque tale with Doyle's twist of mystical conflict.
As one contemporary reviewer noted, it's "an ingenious piece of literature" for fans of Atlantis lore and Verne-style exploits.