The Dungeons of Tonisborg were discovered during the filming of Secrets of Blackmoor, a documentary about the origins of Dungeons & Dragons.
Greg Svenson mentioned Tonisborg on the OD&D Discussion forums, back in 2008:
I built a city, called Tonisborg, complete with a dungeon and a network of catacombs, during 1973 and ran many adventures there and all around the Blackmoor area in 1973 and 1974 using the play test rules for the original three little books and then the published books. Tonisborg was located approximately where Vestfold is on the current Blackmoor area maps, for anyone who is interested.
Greg Svenson, OD&D Discussion, Feb 2008, here
Whereas Blackmoor, Greyhawk, and El Raja Key involved castles with entrances to multilevel dungeons, Svenson described Tonisborg as a city in the approximate location of what would subsequently become the region's capital of Vestfold.
Map of The Northern Marches, labeled with Blackmoor's location, originally posted on Facebook (Dec 30, 2016). The roads leading south and west from The Town of Blackmoor led to the city of "Tonisberg" on early maps of the town. The large city to the southwest of Blackmoor was marked as "Vestfold" on the map in Judges Guild's First Fantasy Campaign (1977).
The makers of Secrets of Blackmoor are including a deluxe, illustrated version of "The Lost Dungeons of Tonisborg" as a high-level Kickstarter reward, (see update #11, update #12, update #13, and update #15 for details).
There were several posts regarding the project on the Secrets of Blackmoor Facebook Page, listed here, as well as an interview on the Idle Red Hands podcast in February, 2019, worth listening here. The most recent Kickstarter update was posted, here.
Daniel Boggs performed an in-depth analysis of Svenson's megadungeon in a series of posts on his blog "Hidden in Shadows" in 2018: The Lost Dungeon of Tonisborg, Tonisborg Part II , Tonisborg Dungeon Part III - Stocking, Tonisborg: The Lost Level of the Lost Dungeon.
Gary Con XI:
I had the pleasure of participating in a Tonisborg session run by Secrets of Blackmoor director Griffith Morgan at Gary Con XI. You can view a recording of the session here and here and here, (I'm the mapper in the foreground on the right).
It was loads of fun getting the chance to explore a classic megadungeon as part of a large group. Our very first battle involved a chaotic melee with a wyvern. We then stumbled into an encounter with undead, before taking a short break, and coming back for more.
Update (December 29, 2021):
"The Lost Dungeons of Tonisborg" shipped in Summer, 2021. See my review.