Doing some organizing in my D&D folder on my computer today I came across this incredible megadungeon map drawn by Giambattista Nolli in 1748. I think I came across this one day while Google image searching for floorplans and recognized it for the masterpiece in dungeon construction that it is. You can visit an interactive zoomable navigable version of the map here (thanks UO!).
I am going to use portions of this for the ruins of ancient Abolon that my 5e Waterdeep campaign players are likely going to be discovering soon under the sewers.
This is just a snippet - the full map is way too big to paste here.