Ever see an ancient map with a dragon-like monster drawn way out at sea?

Here be dragons” means dangerous or unexplored territories, in imitation of a medieval practice of putting illustrations of dragons, sea monsters and other mythological creatures on uncharted areas of maps.

Although several early maps, such as the Theatrum Orbis Terrarum, have illustrations of mythological creatures for decoration, the phrase itself is an anachronism. There are just two known historical uses of this phrase in the Latin form “HC SVNT DRACONES” (i.e. hic sunt dracones, ‘here are dragons’). One is on the Hunt–Lenox Globe (c. 1503–07), on which the term appeared around the east coast of Asia. This might be related to the Komodo dragons on the Indonesian islands, tales of which were quite common throughout East Asia. The other appearance of the term is on a globe engraved on two conjoined halves of ostrich eggs, dated to 1504. Earlier maps contain a variety of references to mythical and real creatures, but the Lenox Globe and the egg globe are the only known surviving maps to bear this phrase. Furthermore, the two maps may be closely linked: an investigation of the egg globe performed by collector Stefaan Missinne concluded that the Hunt–Lenox Globe is in fact a cast of it. “‘Here be dragons,’ [is] a very interesting sentence,” said Thomas Sander, editor of the Portolan, the journal of the Washington Map Society. “In early maps, you would see images of sea monsters; it was a way to say there’s bad stuff out there.”

The classical phrase used by ancient Roman and Medieval cartographers was HIC SVNT LEONES (literally, “here are lions”) when denoting unknown territories on maps.

Wikipedia to the rescue!