Hydrosphere

The Colony on Rigel IV had been founded seven hundred years earlier but for the past six centuries, they had been forced to exist on the bottoms of the great oceans of the planet, kept there by the tremendously potent weapons of the alien star-race which had swept down out of space and wiped them off the land masses of the new world. Kerrel Stevens found himself trapped in one of the Shells, unable to remember how he came to be there, aware only that for some strange reason, he held the secret which could release these people from their terrible existence, but that his memory and all of the knowledge which could help in the struggle against the aliens had been erased from his mind.
In the Shells, he finds what he is seeking - others like himself, different from the people who had become used to this life on the sea bottom, where science had gradually given way to superstition and witchcraft - and this chance meeting provides the key which unlocked the amnesia in his mind. For him, it opened the doorway to the surface of this strange, impossible planet, plunging him breathlessly towards the stars- and the unbelievable secret which spelt destruction for the alien star-race.