A Netherese enclave (flying city) lost beneath the ice in the Frozenfar.
Rumor and Folklore
In ancient times, The Netheril Empire ascended to heights of power mythic by the standards of current Faerûn. Their wizards were able to even build cities that could fly - called "Enclaves", from which the Netheril would dominate and conquer much of the world. These cities fell to earth and into ruin with the event told in legend as "Karsus' Folly".
But before that event, one disappeared somewhere north of the Spine of the World, an enclave called, "Ythryn".
Further Discovery
(As of the 1st of Flamerule, Year of Three Ships Sailing, by the Sewer-side Squad): Exploring one of the outer towers of the enclave that broke off the enclave as it crashed deep into the ice, partial records revealed additional information on this city that was most certainly real beyond its legends: