Bokura no anime
When 15 children (8 boys and 7 girls) are having fun at summer camp, they discover a grotto near the sea. They enter the mysterious location and find a room filled with computers as well as Kokopelli, the man who claims to be the owner. He claims that he is working on a game that involves a huge robot designed to defend the Earth against 15 alien invasions. Kokopelli isn’t able to test it yet so he convinces all the children to sign a contract for what he claims to be a great adventure.
But things begin to get darker once the contracts have been signed. In Bokurano the children now have to pilot the giant robot Zearth one by one in order to be able defeat all the enemies. Kokopelli forgot to mention the energy source of Zearth, the giant robot.