Instructions
Head west to "Fallarbor Town" from Route 144, heading south onto a bridge. Use the HM, or Hidden Move, known as "Surf" to head to a waterfall, later using the HM "Waterfall" to traverse it. Travel up to find the entrance to find one of Terra Cave's potential locations. Return to the bridge and head south if the cave's entrance doesn't appear. Walk west and then continue ahead for the next potential location.
Travel to Route 115 from "Rustboro City," heading northwest to the nearest body of water. Use Surf to travel to the obstruction of rocks blocking the route ahead; head right to an area filled with brush. Travel through the trees and brush to a set of stairs to find the next potential location.
Travel from Rustboro City to Route 116, heaidng east to the entrance of "Rusturf Tunnel." Head southeast from there to potentially find a door. Go into Rusturf Tunnel if the door doesn't appear, heading southeast and then left to find the exit. Head through the exit and walk right to find another potential entrance to Terra Cave.
Head east from "Mauville City" to Route 118, where the entrance to Terra Cave might appear. Use Surf from the nearest body of water, heading east to a set of ledges. Traverse the space underneath the ledges to reach a small berry patch you can clear using the HM "Cut." Search the nearby area to find another potential entrance to Terra Cave if the latter entrance doesn't appear.