Pokémon Gold and Silver/Union Cave

'When you first visit Union Cave, you probably won't be able to use Surf and Strength, which you need to use to get around the entire cave. You will be able to explore the first floor and the middle portion of the first basement. Talk to everyone you see. Some people have very interesting information.'

Take a short detour for TM 39
This cave is pretty simple: just head south, fighting the trainers when you must and picking up items where you can. But if you have the fortitude to handle a short detour, take the stairs on the high ledge to the left. You won't get far in the basement, but you can easily get to TM 39. It contains Swift, a strong Normal attack that always hits and can be learned by most Pokémon.