Apparently, there is a side entrance. Do some backtracking and you should be able to find it. (Source)
However, from what I've been reading recently there is no way to get back to Groudon and this error is a lot like the Lumiose Glitch with X and Y. So firstly, DO NOT SAVE DO NOT SAVE DO NOT SAVE. If the source is incorrect (which shouldn't be the case since two people confirmed it), I cannot stress enough that if you save you will be "royally screwed" as the OP from the source states.
Also, here's another source. I really don't think you should worry, there is a side entrance there apparently. Good luck finding it.
Hope I helped. :)