The best thing about Hoenn is that it had so many little areas like this, where a normal playthrough of the game wouldn't even take you and only adventurous players would ever bother seeing. It's really a shame that later games didn't follow suit with this.
This is one of the coolest examples, but there's also a pretty decent amount of hidden areas in the ocean or behind the bike you didn't choose. And then there's the Regis, which would probably be the hardest-to-find trio in the series without a guide.
