Union Cave connects Routes 32 and 33. You don't have access to much of it without Surf, so just follow it through to the exit on your first visit. If you come back with Surf, you can find a couple more items, trainers, and stronger wild Pokemon.
I T E M S
First Visit:
-Great Ball (x1)
-Potion (x1)
-Awakening
After Surf:
-Hyper Potion (x1)
-Elixer
T H I N G S XT OXD O
-Catch/battle wild Pokemon
-Obtain Items
-Battle trainers
-Head south towards exit and Route 33
* No. of post needed to travel through Union Cave from Route 32 to Route 33 is at least 10.
