As far as I'm aware, you need the badge from Canalave City (the sixth) which can be reached after obtaining the Surf HM from Cynthia's grandmother.
Hope that helps!
- BanetteBrain
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You can use it after winning the Glacier badge at Snowpoint city.
The mine badge.
You cannot go through waterfalls but you can climb them with the HM waterfall and a Pokemon that can learn it... you will also need the corrosponding gym badge
To find HM 05 Defog, go to the Great Marsh and pick up the pokeballs on the floor until you get it. You'll need the Relic Badge and the Fen Badge to use it. (AKA, Gym badge 4 and 5)
The 7th badge.