Although just a huge and fairly featureless hole in the ground, Ideon has sacred importance in mythology as the place where Zeus was reared by his mother, Rhea, to save him from the clutches of his child-devouring father, Cronos. (Some also believe it's where he died and is buried.) Ideon is on Mt Psiloritis about 15km from Anogia; it's a 1km uphill walk along a rocky path from the parking lot to the entrance.
Ideon was a place of worship from the late 4th millennium BC onward, and many artefacts, including gold jewellery and bronze shields, statuettes and other offerings to Zeus, have been unearthed here. The rail track used for these archaeological digs is still here.
In winter (and sometimes as late as May) the cave entrance can be blocked by snow, in which case it's easy enough to climb over the fence. However, for safety reasons, avoid clambering on the snow.