It is doable, I've done it about 3 times. Make sure you're healed and hug the walls. It takes alot of work, but eventually you will find the stairs.
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In Pokémon Red, Blue, and Yellow version you have to go through the Rock Tunnel. The best way to get through it is to use Flash.
Flash is needed.
You can actually go through the cave regardless of having the HM Flash. Simply use the little light they already give you and slowly make your way through. Also you use Flash if you really need it, to get Flash go to route 2 and use cut on the tree go north to a big building and talk to the scientist say Yes to his question you will get Flash. Use flash and now you can see through rock tunnel.
Get a Pokemon that can learn flash and put them in your team , teach them flash and go through following the ladders like Mt Moon
Almost any electricity or fire pokemon, otherwise pretty much only the occasional weaker pokemon. I believe Ratata MIGHT be able to. If you are looking for it for Rock Tunnel there is a guide on IGN.com for how to get through without Flash.