Quite interesting - Mayan day/nights and earth periods

[3DMonkey]

underground cities are a curious case. what was the reason they were built ?

to keep a portion of south american population continue the work they are doing, at the wake of the future invasion and conquest by spanish or others ?

to be a place for the soon-to-start 4d civilization to live while the 3d population fades out ?

to be a place for the harvested and dual activated to live out their life and start the 4d society when the surface gets uninhabitable ?

It doesn't need to be a geocentric reason.