Tunnel

Tunnel —

Distance —

Speed —

Clearance —

Best —

How to inspect a tunnel

One button — hold Space, ↑, or the mouse to climb. Let go and you sink. That is the entire control scheme.
The rock is not negotiable. Neither is the pillar in the middle of it.
You go faster the further you get, and the tunnel gets narrower. Both of these are on purpose.
Every 800 metres you enter a new tunnel. The names hold up for a while.
Every route has a number, and its link goes straight back to that exact route.