Pokémon Red and Blue/Rock Tunnel

'Rock Tunnel is the only way to get from Route 10 to Lavender Town. Unlike the caverns beneath Mt. Moon, Rock Tunnel is pitch black, and you must rely on the Flash technique (HM 05) to light your way. Use the labeled maps to follow the ladders through the tunnel to the exit. The tunnel has two levels, but there really is only one main path.'

Floor 1: into the dark
The wild Pokémon in Rock Tunnel tend to attack more often than their above-ground cousins, and if you're being attacked every few steps, it may take you awhile to reach medical attention. For that reason, it's a good idea to bring either a Pokémon that knows Dig, or plenty of Escape Ropes just in case you need to make a quick getaway. If you don't have either of those, this would be a good time to try Repels, which keeps weak wild Pokémon at bay. However, Pokémon who are stronger in levels than you lead Pokémon can still ambush you.

If your starter Pokémon was Bulbasaur or Squirtle you'll have a very easy time against the Rock and Ground-type Pokémon that lurks here. There are also some trainers with Water-type Pokémon so Electric-type Pokémon does good against them. Bring a few /, Potions, and maybe a  or two to make sure you can get through in one trip.

The cave is very dark and your screen will turn almost black so you won't see anything except for the walls that are bluish. The trainers aren't too tough here but you can accidentally run into them if you're not careful. You'll have to use Flash (HM05) if you want to see where you're going in here. It's possible to proceed without it, but if you do, hope you recognize where everything is. If you want to avoid a battle, walk a path that's far away from the trainer or that's out of the trainer's line of sight.