On the 6th of July 1734, when off the south coast of Greenland, a sea-monster appeared to us, whose head, when raised, was on level with our main-top. Its snout was long and sharp, and it blew water almost like a whale; it has large broad paws; its body was covered with scales; its skin was rough and uneven; in other respects it was as a serpent; and when it dived, its tail, which was raised in the air, appeared to be a whole ship's length from its body.
-- Hans Egede, Norwegian missionary, later bishop of Greenland [source: AMNH]
In 1817 and 1819, more than 200 residents of Glouster Harbor, Massachusetts, recounted seeing a giant creature that resembled a serpent. "The Great Sea Serpent," an 1892 book by professor A. C. Oudemans, describes more than 200 reports of unknown sea creatures. But then thousands of people over the years have reported sighting the Loch Ness Monster, aka Nessie, yet no scientific evidence for its existence has yet been found -- and not for lack of trying.
What are scientists to make of such creatures? On the one hand, we still discover strange new sea fauna over time, and by some estimates as much as 95 percent of the ocean's lowest depths remain unexplored. We know, too, that some creatures that resemble sea monsters, such a giant squid and oarfish, spend most of their lives in deep or deep-ish waters, entering the shallows or washing ashore only when sick or dying. So it seems reasonable that remarkable creatures could yet exist, whether encountered by sailors or completely undiscovered.
But admitting the possibility that we have not seen all that nature has up her watery sleeve is not the same as conceding the existence of creatures that defy the laws of physics, chemistry and biology. Scientists may not be able to comment on the fanciful, and might find it difficult to disprove the existence of a thing, but they certainly can apply known principles to establish boundaries on what might lurk undiscovered beneath the waves. After all, the first coelacanth (Latimeria chalumnae) was discovered as recently as 1938, and the Megamouth shark, caught in 1976, was even more recent, but both conformed to the basics of oceanographic physiology [sources: Smithsonian; Western Australian Museum].
Such answers are the best we can expect for now, until we drain the seas or until some rough beast emerges from them to announce its presence in no uncertain terms.