In the Fallout Equestria: Shades of Grey fanfiction, the DERTA was a Prench research center located under Big Mountain in the Crystal Mountains Range.
Before the war, Big Mountain was a small Prench enclave between Equestria and the Crystal Empire. As the events from the show unfold (Nightmare Moon's return, Discord's chaos, the changeling invasion...), the Prench government, unhappy of learning such events in the newspapers, decided to turn this abandoned salt mine into a listening station, using the research center as a cover. They soon come to realize the place also allowed them to conduct studies without fearing being spied on.
During the WarEdit
After the Bombing of Prance and the invasion that followed, Big Mountain became the last 'free' Prench territory. Equestria took the center under her wing.
When the bombs fell, the supposedly impervious shield dome protecting the place failed spectacularly, cutting the power and flooding the tunnels with radiations. There is no known survivor.
Given the location of Big Mountain and the care given to making the place impervious, the DERTA remained locked and forgotten for at least two centuries. It wasn't before Spring - and Crowneigh before her - managed to locate and acquire all four security keys that the DERTA got finally opened.
The DERTA is located inside Big Mountain, which is part of the Crystal Mountains.
The place is remote and devoid of settlements.
Behind the ScenesEdit
The DERTA is loosely based on the Big Mountain Research and Development Center from Fallout New Vegas: Old World Blues and the real-life Cheyenne Mountain nuclear bunker.