The Guardian Base Tunnel

The Guardian Base Tunnel is a ten-mile-long tunnel that connects Guardian Headquarters to the Lake Base. There is a hollow for people to rest, which contains a portal that leads back to the main base. The tunnel can block radio waves and disrupt communication, preventing enemies from remotely detecting it.

Guardian Era
When Stardown was paralyzed by Dr. Sykk, Tails, Amy, Kervo, and Mark carried his bed into the tunnel. It was discovered by Armaul, who had come to pick up Stardown, but he was beaten by Sonic the Hedgehog.

Congregation Era
After emerging from a portal in the tunnel, the Minor Villains used it to access the Guardian Headquarters.