One of my favorite mysteries in cryptozoology concerns the alleged giant fish of Alaska's Lake Iliamna. An illustration of how useful Google Book Search can be to historical research is this bulletin from 1906. (The original spelling and punctuation is preserved in this excerpt.)
Bulletin By Geological Survey (U.S.): "Iliamna lake, largest in Alaska, a few feet above sea level, about 60 miles long and from 15 to 25 miles wide between Bristol bay and Cook Inlet. Named Shelekof by the Russians as early as 1802 but now universally known by its native name Iliamna, locally pronounced Lamna. According to Martin, Iliamna is 'said to be the name of a mythical great blackfish supposed to inhabit this lake which bites holes in the bidarkas of bad natives.' A Russian map of 1802 calls this Shelekof while Clark lake, supposed to to have been discovered in 1891, is shown and called Ilima lake."