The Ones Lost; Fabled, revered even, as if legend. Because only one met them, long ago.

The Empress was forth-giving with their accumulated scientific knowledge that she saved from oblivion, but she was always tight-lipped about them otherwise. Her ponies only ever got an inkling as to why, and little more from her.
The only ponies that she was frank and open about them to; were the other princesses, her family of sorts.

Geology yields new developments, understandings, and often surprises... archaeology is one of them

Whether this is alt-universe/canon or not, I haven't decided. Maybe, just a what-if? Either way, this takes place after parts 1-3 by a few centuries.