This all seems so familiar..
The Wind Tunnel (大風洞, Dai fūdō, Large Wind Tunnel) is an area accessible from the Abandoned Factory by getting eaten by the large red monster in the sewers.
The Wind Tunnel is a large open area resembling Yume Nikki's Windmill World, filled with small three-bladed pinwheels and a few large concrete structures scattered around the area.
After being swallowed by the large red monster in the Abandoned Factory, Urotsuki will find herself under a gigantic concrete structure surrounded by spinning pinwheels. By heading east upon entrance, you can find an NPC that looks like a person wearing a green shirt standing in the center of a circle of fern-like plants, who will take you to the Deserted Pier when interacted with. Another ring of pinwheels to the south of this green-garbed being surrounds a group of stone buildings, one of which is able to be entered.
The building's interior is very dark with an eerie, windy soundtrack. Going to the left or right, Urotsuki will find some blue lights hanging from chains. Heading north from the lights, there will be more lights and the same type of pinwheels found outside of the building. At the top is a door marked with an ornate pattern above it that leads out into the Radiant Ruins.
- This area and the event resembling Big Red leading up to it are likely a callback to Yume Nikki's Windmill World and the event that would take you there.