So as I'm sure many of you have realized, when looking for unexplored worlds, you'll think you have a good one, only to discover that your "50%" explored world is actually mostly unexplored sky. Ask yourselves then, how do we solve this? At the moment there is no incentive (with the exception of knowing you made that world more explored) for the immensely boring task of flying around to clear the skies. I understand that if the same amount of XP was given to explore the skies as is given for exploring underground that the system would be greatly abused (people getting high ranks by doing basically nothing). The thing is that by taking the time to explore the skies though, often you sacrifice the ability to obtain any desirable items. So here's how I think we should solve this:
1. Airships: airships would be floating airship wrecks (lore: they never fell after the apocalypse and would've drifted aimlessly for a long time.) There could be some crashed on the ground too. Mostly they would've been escape types (people trying to escape the brains by going up and moving away instead of going underground), Warship types, transports, Sky pirates (maybe), Air-liners, etc. Some would have chests etc, some might be more dungeon-ish and have been taken over by brains, maybe about 5-10 per world depending on size.
2. The other idea is to award 1xp per area of sky explored, this reduces the ability to abuse it and get levels fast, but provides enough incentive for people to actually clear out the skies a bit.
3. Aviator achievement: similar to explorer ahievement, given for exploring "X" many areas in the sky.
4. In The Moon's Eye achievement: (low level achievement, reference to what is commonly accepted as the first sci-fi film) achieved like the Spelunker achievement, but for reaching the top of the world, would grant very little XP, as this is not a very hard or time consuming endeavor.
Edit: Grammar and clarity Edit2: More grammar, clarity and formatting.