Ancient Greeks, the founding and fall of Rome, the Egyptian empire, lost Atlantis. Cave paintings, Easter Island, Stone Henge, Chaco Canyon, Scara Brae. Ley lines, the Devil's Sea, the Bermuda Triangle. King Arthur, Odin Allfather, Mother Earth (Gaia). Aliens, unicorns, Big Foot. Gilgamesh, Marco Polo, Aragorn.
All of these things conjure stories and swirling mystery at the mention of but a few words. There are all kinds of legends and myths, tales of lost cultures. Relics, ruins, and artifacts litter the Earth with the drifting changes of human history. How many of them are truly the makings of mere humans? Will we ever really know?
What kind of ruins dot the landscape in your world(s)? What unsolved and unexplainable phenomena complicate your tales? What legends, myths, and folktales are on the lips around campfires?
These things don't have to be grounded in fact to be believed. Maybe various myths were collected by an old language professor and inspired him to weave his own story of ancient magic and fallen kingdoms. Perhaps his telling led to countless other worlds being crafted from countless others' imaginations.
An old farmer breaks his plow on an enormous bone hidden under his unremarkable field. Upon digging he finds...what? A dragon skull, a long-dead giant, a whole graveyard of fallen titans?