[SPOILER ALERT]
Ok, I'll try to give some hints without revealing too much! :D
Basically, the alien coins are the key for three out of four endings.
Apart from the "normal" ending where you escape through the well, you can leave after collecting all of the coins; or access (thanks to them) an otherwise inaccessible place, where you will face, uhm, someone.
At that point, you will have two more options on how to end your quest: one blatant, the other one more... creative! :D
Hope this will help. ;)