The Mine is the 3rd world of the game, it opens up along with The Skyline once you beat Outskirts. Some people highly suggested you complete this world before skyline. Beating the boss in The Mine gives you one of the keys to the Volcano Gate
When you enter, Filo sees an Ark Seed that creates Generi Me, a slightly unstable clone. After walking a bit farther you can see Asimov resting as a yellow moon, attracting gas. After this Filo and Generi Me must copy this ability, in level 2 you see the worms again, and use them to mostly travel around.