Pokemon Emerald Optional - Sky Pillar.
If you want you can head back to the Sky Pillar to catch Rayquaza. However he is LVL 70 so unless your Pokémon are around LVL 60 or you plan to use your Master ball it will be extremely hard to catch him.
Head upstairs but notice that here are a lot more rocks than last time.
You need a Mach Bike now to reach the top.
Get a running start and get to the bottom.
From here get ready for a tight turn.
Head to the left.
To the right.
And up again.
Head to the next floor and get ready for a really tight turn.
Head left then turn straight up at the rock.
Next back up.
And fall at either the second or third crack.
Now head to the top where Rayquaza is.
Save before the fight, beacuse if something goes wrong you will not be able to catch Rayquaza again.