Inside the tunnel.
I counted 15 sections of pipes, each 2.5m and a shorter one at the entrance, total c. 40m / 130ft.
The tunnel is straight, goes very slightly up at the beginning and then horizontal for the last third. The temperature increases quickly.
The vault is made of concrete (except the stones you can see here at the entrance, and bricks at the very end). The more you progress the more mud has accumulated on the ground (especially towards the end).