Pokémon FireRed and LeafGreen/Rock Tunnel

The Rock Tunnel Connects Route 10 to Lavender Town. To get through, you need to have a Pokémon that knows the HM05 Flash. There is also a Move Teacher who will offer to teach Rock Slide to one of your Pokémon on the second floor.

If you picked Squirtle or Bulbasaur as your starter and leveled it up decently, you'll have little problem getting through. Those who picked Charmander as their starter will struggle though, as the rock types here are super effective against fire types. Pick up as many super potions as you can in Vermillion City, as you'll be using them. Worry about getting to the other side and come back for the items/Pokemon later once you have a stronger team.

On the second level, in southwest corner, there is a guy who will teach your Pokémon Rock Slide.

Items

Max Ether (North of Picnicker Sofia and Hiker Dudley)
Revive (B1F; southwest corner)
Escape Rope (Southeast of Hiker Lucas)
Pearl (North of Picnicker Dana)
Repel (1F; northeast corner)

Pokémon Found

Ground
FireRed	Level	Encounter Rate	LeafGreen	Level	Encounter Rate
Zubat	15-16	30%	Zubat	15-16	30%
Mankey	16-17	15%	Mankey	16-17	15%
Machop	16-17	15%	Machop	16-17	15%
Geodude	15-17	35%	Geodude	15-17	35%
Onix	13-17	5%	Onix	13-17	5%

Battles (In Cave)

	Name	1st	2nd	3rd	4th	Money
1	Ashton	Cubone 23	Slowpoke 23			1104
2	Winston	Slowpoke 25				1200
3	Martha	Oddish 22	Bulbasaur 22			440
4	Steve	Charmander 22	Cubone 22			1056
5	Allen	Geodude 25				900
6	Eric	Machop 20	Onix 20			720
7	Lenny	Geodude 19	Machop 19	Geodude 19	Geodude 19	684
8	Oliver	Onix 20	Onix 20	Geodude 20		720
9	Lucas	Geodude 21	Graveler 21			756
10	Sofia	Jigglypuff 21	Meowth 21	Pidgey 21		420
11	Dudley	Geodude 21	Geodude 21	Graveler 21		756
12	Cooper	Slowpoke 20	Slowpoke 20	Slowpoke 20		960
13	Leah	Bellsprout 22	Clefairy 22			440
14	Ariana	Pidgey 19	Rattata 19	Rattata 19	Bellsprout 19	380
15	Dana	Meowth 20	Oddish 20	Pidgey 20		400