Go all the way back until you reach the first fork. Take the path above and you will come to a brick wall. Use Gil's bombs, which you just obtained, to destroy the bricks and open a passage for you. Continue, and you will arrive at the lake under the swamp. Go across the lake, exiting to the left and into the next room go down. Proceed to the left and, in the room just after the pipe, go down again. Exit to the left. You will come to a pond inside the mine. Get out of the water and let yourself fall into the small precipice on the left. Go left again and take the passage on the left side of the precipice to reach the pond with the chest at the bottom. Switch to Gil to get it. Inside you will find the Blue Ring. Then follow the path you have done in the past to reach Shades, but this time, after having climbed the second rope, go left and enter the cart, to reach a chasm on the left side of the mines. Halfway up there is a passage on the left: enter it. You will find yourself in another area with brown earth walls. Kill the frogman, switch to Gil and enter the water. Ignore the narrow tunnel that goes up and jump out of the water on the other side. Exit to the left.
You will arrive in an area full of ice and snow that is much larger than the one you crossed at the beginning of the game. Go down and continue to the right always defeating the enemies, ignoring for now the paths that go down. You will come to a screen with a door. Climb the snowy wall above it with Bop-Louie's suction cup (fear not, it also works on icy walls) and above you will find a chest with another Life Container. Go back down and enter the door. You will find yourself again at the entrance to this area. Repeat the path as before and enter the second path that goes down, the flooded one, switching to Gil. At the bottom, continue to the right and exit the water. Switch to Freeon-Leon, as he is the only one who can walk on ice without falling. Go down and take the narrow passage on the left. To defeat the enemies you encounter you will need to temporarily switch to Shades or Bop-Louie and use their special weapons, because you cannot jump here. Then press the switch you will find at the end of the path to make the snow fall and make the area below safe. Go down and take the passage to the right by climbing with Bop-Louie's suction cup. Then, switch back to Freeon-Leon. All you have to do now is follow this path until you meet a boss.
This boss is a trifle. The only difficulties are that the boss makes a jump at regular intervals and that for the battle you will have to use Freeon-Leon, whose jump performance is not very good. For the rest, however, the boss is similar to the first one you met. Jump on its head to make it spit the ball and throw it at the boss. Repeat the process 4 more times to defeat it.
Continuing to the left, you will find a chest with Freeon-Leon's special weapon, the Frozen Breath, which allows you to turn enemies into blocks of ice. Your group is thus fully empowered. It is time to look for the keys.