Diglett's CaveFireRed&LeafGreen
"A shortcut tunnel between Vermilion City and Route 2 near Pewter. Nothing dramatic, but it saves a lot of backtracking once you need to return to northwestern Kanto."
DIGLETT'S CAVE MAP
Pokemon Encounters
Almost nothing but Diglett at 95%, levels 15 to 22. Dugtrio is the 5% prize that saves you from grinding an evolution. Quick Ground-type coverage if you still need it.
Diglett
Diglett's Cave Guide
It connects Vermilion City to Route 2 and has fast encounters. Diglett are level 15-22, Dugtrio level 29-31. If your team is around level 25-30, Dugtrio gives decent experience and the cave is short enough that you can loop quickly between the exits and a Pokemon Center.
Yes, but only at 5% encounter rate on both floors. You'll run into mostly Diglett. Dugtrio show up at levels 29-31 and have Arena Trap, which prevents you from running away. If you just need one for the Pokedex, use a Repel with a level 22+ lead to filter out Diglett.
Depends on what you're after. Repels block encounters below your lead Pokemon's level, so a level 20 lead in Rock Tunnel filters out the common level 15-16 Zubat while still triggering rarer spawns. If you're grinding, skip them. If you're just passing through, they save time and healing items.
Only Rock Tunnel is completely dark without Flash. Every other cave in FRLG is fully lit by default. Flash itself comes from Oak's aide on Route 2, and you only need 10 Pokedex entries to claim it.
Escape Rope warps you to the last entrance you used and costs 550 at most Poke Marts. The move Dig does the same thing for free if a Pokemon on your team knows it. Both work in any cave or dungeon in the game.