Tunnels of Madness
The Tunnels of Madness are a series of tunnels, mines, caves, and rail lines located under the spawn of the overworld. It supposedly has no log blocks and is estimated to be from 2018 or 2019.
It has been hypothesised that the tunnels are a sort of Minecraft Backrooms. These tunnels exist at Y=5-12, and can vary from 2x2 to 2x1 shafts, the length, the amount, and area these tunnels cover is currently unknown, but there exist many entrances and exits, with one being a random mine at spawn and another being the mines at Vernacia. More information must and will be gathered.
There also exists a 1x1 diagonal shaft, which seemingly leads to nowhere except a small lava pool. Why it was built in such a difficult way is still unknown, as are the origins of the other tunnels.