Like Route 1, this route only contains one exit. When you arrive, you'll find a small pool of water with a skull swimming in it. Shoot the skull as you dive in and hop out on the left side. Several slowly falling enemies will be dropping to the ground before bouncing back up again. These enemies are very easy to kill, and you can walk back and forth in this area to generate more of them and increase the amount of money you have. Try to collect at least $20000 before you leave.
As you go further to the left, an insect will approach you, followed by a enemy that slowly rotates in mid-air. Remove it and then jump up through the platforms to the screen above. You'll see three slowly falling enemies drop to the bottom. Getting up to the first ledge can be tricky as you have to bang your head on the ceiling a few times first. Then jump up to the top of the ledge and begin climbing the cliff wall to reach the next screen.
A guy will be walking back and forth. When you approach him, he informs you that there is gold in "those mountains over yonder," and that you can exchange them for money on Route 22. The description of this location is a little vague. Continue to the left and you'll see a large "WELCOME" sign. Go to the left side of it, and a shop icon will drop from the sky. Jump up to touch it and visit the store.
On your first visit to the store, you should pick up an Aqua Lung for $7000. You should also buy at least one Shop Call item if you don't have any (you start with one), and refill your supply of Ultra-Food if you have used any. If you're gearing up for the first medallion battle, you should consider buying a Guard Crystal and a Big Shot as well. When you ask to leave the store, the shop keep will always give you the opportunity to buy back items you don't need, but at a $2000 discount.
With the Aqua Lung in your possession, you can now investigate what the guy meant when he said "those mountains over yonder."
Diving through the water
Equip the Aqua Lung and dive into the water. Swim down to the next screen, and continue on to the screen after that. In the water, you will encounter mines which float up and down a bit, and which explode like the heat seeking missiles. You'll also encounter frog monsters along the bottom of the water as you swim to the left. Then climb back up through two screens to return to the surface.
Hop out of the water, and watch out for the little creature that darts towards you as you climb out. You can unequip the Aqua Lung now if you like. More slowly falling enemies occupy the area as you head left. When you reach the end, drop down to the screen below. You'll have to shoot your way through the rocky tunnels. As you do, you should discover hearts in some of the rocks. These hearts will always return if you leave and come back to this screen.
Keep going to the screen below. Along the floor are some more rocks you can shoot out to the right. Behind these rocks are a set of gold bars. If you only shoot the bottom two rows of rocks, you can get at least four gold bars each time, and you can get a few more if you shoot out the top row. You can then leave and return to this screen to reset the rocks and the gold bars. Keep shooting rocks to obtain more gold bars until you reach the maximum of 99.
Once you have as much gold as you can carry, your only choice is to retrace your steps, climb back up to the top, and return to the water on the right. Don the Aqua Lung again and swim back through the water to return to the welcome sign, and then make your way back to the route entrance. Remember to push + to drop down through platforms.