Celadon city.
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You need the rainbow badge which you get a celadon city Hm cut is needed.
The 8 badges for Pokemon Red are: Boulder Badge, (Brock) Cascade Badge, (Misty) Thunder Badge, (Lt. Surge) Rainbow Badge, (Erika) Marsh Badge, (Sabrina) Soul Badge, (Koga) Volcano Badge, (Blaine) and Earth Badge. (Giovanni)
you sneak into it and Get in there go get a rode and do it
To move a big boulder you need obtain the move Strength. To use the move outside of battle you need the Rainbow badge.
You can obtain the 8th gym badge (the earth badge) in fire red from Giovanni in Viridian City.