The Indie game Titan Attacks! comes with various Steam achievements. One is called EARTH and you'll get it for completing the EARTH levels. I did this several times now, but still haven't unlocked the achievement. Is it broken? How can I earn this achievement?
Yes, all of the "Section Complete Achievements" are broken currently, like Mars, Saturn, Earth, Titan, etc.
The developer is working on it.
There are no hidden levels and you don't return later by natural progression, so you might have to replay the whole section to get the achievement. We're going to have to see how Cas is fixing it and if it will grant the achievement after you have already completed those sections.