Earth's level has to do with the number of Walkr friends you have. If you want to upgrade Earth to level 7, you must first have at least 80 Walkr friends to then unlock a Wonder of the World. Please go to Menu > Social > Ranking > Search for Friends to find friends and add them to Walkr!

(For every 5 friends, your Earth's population will increase by 1. Earth's maximum population in Walkr is 16.)