No, actually -8500 DR was when elves had a city their and had burial tomes (which I would have always though elves used fires or allowed the bodies to rot, but that's what the books say). They left in -1100 DR, but the dwarves had already built the Underhalls of Melairbode by then. -50DR was when a Illuskans settled where Waterdeep now is, started with farms, became a fortress in 882DR and developed into a city in in 952/974DR (depending upon source and definition).
The Yawning Portal was founded by Durnan in 1306DR.
You can see a good timeline of the Undermountain in this post;
https://www.facebook.com/search/str/Ultimate+UnderMountain+timeline/stories-keyword/stories-public
And you can find a good history with dates for the City in this;
http://www.dmsguild.com/product/248971/Waterdeep-Primer
In short, there were thousands and thousands of years of history, conflict and probably exploration of the area, both above and below ground.