Clash at Demonhead/Route 40

Description
Assuming you arrive from the left exit, you are immediately attacked by men with guns. Remove them and advance to the right until you see a well and a statue with a heart between two platforms. In order to use the Magic Stone and reveal the Sword of Apollo, you must actually stand on the platform where the heart is located. There are two ways to accomplish that. You can use the Force, Micro-Power, and shrink so that you're small enough to get in there. Or you can stand on the ledge next to the head of the statue, and push to drop down below and access the heart.

Either way, once you're standing there, select the Magic Stone on your item menu and activate it. The statue head will disappear and reveal the sword. Jump up to collect it, and your power meter will increase by two bars. At this point, there is very little reason to proceed through the rest of this route, which involves equipping an Aqualung, and swimming below some water in order to reach the right exit.

Instead, your next goal should be the Devil's Tower along ../Route 39/. In order to get there, you can teleport if you bothered to enter that route before teleporting back to ../Route 6/ earlier. Otherwise, you can actually jump in the well, which will transport you back to the underground tunnel below ../Route 15/. From there, you can return to the bottom of the well by walking to the left, and using an Aqualung to swim through the water and access the tunnel that leads back to ../Route 33/, and take the right exit to reach Route 39.