How about a bottomless, critter-spawning lake?
One that may or may not be sacred, and may or may not lead to the Underworld?
From Wiki: "In classical Greece, Lerna[1] (Greek: Λέρνη) was a region of springs and a former lake near the east coast of the Peloponnesus, south of Argos. Its site near the village Mili at the Argolic Gulf is most famous as the lair of the Lernaean Hydra, the chthonic many-headed water snake, a creature of great antiquity when Heracles killed it, as the second of his labors. The strong Karstic springs remained; the lake, diuminished to a silt lagoon by the nineteenth century, has vanished.
Lerna was one of the entrances to the Underworld, and the ancient Lernaean Mysteries, sacred to Demeter, were celebrated there. Pausanias (2.37.1) says that the mysteries were initiated by Philammon, the twin "other" of Autolycus. At the Alcyonian Lake, entry to the netherworld could be achieved by a hero who dared, such as Dionysus, who, guided by Prosymnus, went that way in search of his mother Semele. For mortals the lake was perilous:
"There is no limit to the depth of the Alcyonian Lake, and I know of nobody who by any contrivance has been able to reach the bottom of it since not even Nero, who had ropes made several stades long and fastened them together, tying lead to them, and omitting nothing that might help his experiment, was able to discover any limit to its depth. This, too, I heard. The water of the lake is, to all appearance, calm and quiet but, although it is such to look at, every swimmer who ventures to cross it is dragged down, sucked into the depths, and swept away."
—Pausanias, 2.37.4.
http://en.wikipedia.org/wiki/Lerna