This is likely
Isn't that really "THe Big Secret"?
A Global Ice Age civilization
Wrapped around the Earth on a different equator
400 feet lower ocean levels
A seafaring civilization that centered on ports
An impact (comet/meteor)
Massive climate change
Flooding everywhere as the Ice Caps melted
The entirety of human existance is destroyed, buried, and swallowed by the rising seas
We, the descendants of the survivors, left wandering the Earth, trying to put the clues together.
New temples built on the ruins of 11,800 year old temples.
The truth is concealed, except for maybe in Antarctica?
The seafarers would've only noticed the change at the ports. The network lives on the surface of the waters.