Dark Cave's south side is on Route 31, and you can access both as soon as you finish the catching tutorial on Route 29. Most of the items on the south side and the Route 46 entrance are accessible only after you defeat Falkner and Whitney and can use rock smash and strength. After you defeat Morty, you can use surf to visit a small part of the north side and catch the Pokemon there. The whole cave becomes accessible when you enter it from Route 45 after defeating Archer in the radio tower.
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