How do you get to Mirage Tower in Pokemon Ruby?

1. Get past Team Aqua / Magma in Lilycove City and obtain the HM for Dive from Captain Stern before continuing to Route 126. Use Dig in the small patch of dry land.

2. In Route 126 take the southern exit to Route 127.

3. Go south-west past the entrance to Meteor Falls, through some long grass, to dive and enter Mirage Tower.