The St Peters Twins tunnel is a twin tunnel system that runs parallel with each other and exits at the same location. Both entrances run for ~1.3kms. The right circular side only stays cylindrical for a while then turns box-shaped till the end.
You can then go down the other side into the Square Enterance.
The cops are hot on graffiti in the area so be careful. We were questioned by 2 officers before discretely disappearing down the hole for another awesome tunneling adventure…