These fleshy looking maze-like tunnels resemble a womb. There are multiple small rooms throughout the tunnels, some of which contain Fetuses. It can be accessed from the Pit & Temple.
Players usually find this location by going through the room with the sumo ring, though the Lips and Television can link the player directly inside of them.
Often times players will come across a single fetus while walking through the Flesh Tunnels. This fetus will then turn around and run down the tunnel until it disappears into a wall. Following it and walking a little bit further takes you into a room full of fetuses, all bouncing and making noise.
It's possible to encounter more than one fetus when exploring the tunnels.