Here is the full list of all 20 Really Big Sky achievements.
LOTS AND LOTS OF SMALL THINGS
GIVE A HOOT
HAVE SKILLFUL BUDDIES
BE LIKE HARRY STAMPER
KILL BIGGUNS
DIGGING HELPS
NOT TOO IMPOSSIBLE
BE RICH AND THEN NOT RICH
GET A LOT OF KILLS
USE THE BOOMER
FIGHT BOSSES
DON'T KILL DEAD THINGS
CLOSE ENCOUNTER
GET MULTIS
LAST A WHILE
GET 10 LOTS OF HIGH SCORES
NUKE ITS BROTHER
REMOVE BOSSES
BE RISKY
GET SOME KILLS