Using the Cut ability, chop your way through to Route #9 from Cerulean City. Check into the Pokémon Center near the entrance to the Rock Tunnel, then plunge into the underground passage. It emerges on Route #10, a very short route to your next destination, Lavender Town. This might seem like a roundabout way to get there — but it's the only way.
The encounters here are annoying, but at least many of them can be avoided and there's a Poké Center at the end. Make sure you have your Cut-knowing Pokémon with you, or the HM and a Pokémon who's willing to learn.
Before you enter the tunnel, hack aside the top bush and search beneath it for a Super Potion.
Voltorb appear entirely like Poké Balls. Despite being Electric type Pokémon, they learn no Electric attacks naturally without the aid of TMs. Rather, they learn Normal attacks including the life sacrificing Selfdestruct. Once learned, this move should only be used in an emergency.
The Magnemites in Yellow aren't as fast as their Voltorb cousins in Red/Blue, but their mix of strong Electric skills and Normal attacks are more versatile than the Normal-only skills of Voltorbs.
Power Plant
The Power Plant is visible from Route #10, but you can't reach it just yet. Sometime later, you'll obtain a Hidden Machine that will enable you to use the Surf technique. You'll be able then to travel along the stream that flows past both Route #10 and the Power Plant. Until then, you'll just have to keep your curiosity in check.
Voltorb
Magnemite
360
Fearow
Raticate
Sandshrew
Nidorino
Nidorina
Slowpoke
Krabby
