Can you access the cave in the room you battle Wally?

Question

Walk on the right side of the bridge and you should see a cave in the corner. Is there a way to go there yet?

Answer 1

No.

You can't.

Source: Serebii, experience and Pokémon Omega Ruby & Alpha Sapphire: The Official Hoenn Region Strategy Guide

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However, it is almost certainly not there for show. It is likely going to be part of an Event at some point in the future. (Volcanion/Hoopa?)