Deep Ones

Though there is not visible evidence, proffessor George Cavendish belived that the deep ones were the 'cause' of Innsmouth syndrome. He believed that his wife Anna claimed to have conceived a child by one causing it to be 'blessed by the sea'. Although Charles Reed never sees the child, he is impressed by the professor's investigation into the Esoteric Order of Dagon. Given Reed had heard the echos of the conversation the couple had and the fact that he did not further question the matter, he either believed the professor implicitly or saw the threat the Order posed regardless of his marital problems.

Deep Ones and the Order
Quite what the Order's beliefs are opaque at best but they hold the the esoteric creatures of the deep as holy beings of the sea and/or Cthylla as divine. The order either pressures or demands that members of the cult mate with the deep ones and thus give birth to Innsmouthers, beings they see as superior to standard humans. It should be noted that the only known case of this was with a human female, so in all likelihood the deep ones are trying to spread the influence of Cthylla by bloodline.

H.P. Lovecraft
Lovecraft made use of the deep ones in a manner similar to the way they are presented in the sinking city; he seldom if ever made direct reference to him but he made the idea certain that the 'Innsmouth look' with a result of interbreeding with deep ones. As this is doubtless an inspiration for this, the deep ones are almost certainly real.