The Deep Ones are a race of frog-like, ocean-dwelling creatures with affiliation to the Great Old Ones, who created them, and an affinity for mating with humans.
The Deep Ones were one of the many monstrosities created by the Great Old Ones during Earth's primordial age, and dwells on a South Pacific island, dubbed as Marsh Island after founded by Captain Obed Marsh in 1830, located near Zara's Kingdom. A small group of these creatures were encountered by a sextet of exemplary Englishmen led by Captain Corcoran, who'd previously served aboard the HMS Pinafore, in which Corcoran amazingly taught them to sing (albeit only in thick, bubbling, inhuman voices) at a command performance held for Princess Zara.
The Deep Ones first appeared in H.P. Lovecraft's The Shadow Over Innsmouth (1936). The lyrics sung by the Deep Ones describe what happens to the narrator of The Shadow Over Innsmouth in the style of Gilbert & Sullivan.