For perhaps thousands of years, local Cappadocians retreated underground when enemies approached. Their subterranean city, illustrated here, was rediscovered by accident. YASIR999/WIKIMEDIA/CC BY-SA 4.0
The Hopis claim their ancestors emerged from the underworld and other Pueblo and non-Pueblo Indians have similar legends.
Perhaps they were referring to an underground city of some kind.
We actually know so little about what’s going on under our feet and deep in the ocean.
Some claim there are advanced, hidden civilizations that escaped the surface wars or even mysterious cryptids live underground and emerge from certain openings.