I've been trying to find this book (obviously unsuccessfully) for at least six or seven years. I read the book only once and sometime about 9-10 years ago; hence my memory of it might be a bit off on certain things.
Aliens, who are mostly (if not altogether) unknown and unspecified, threaten the Earth with partial extinction unless a certain set of keys (not literal keys, of course) are collected and brought to some sort of alien pyramid. The pyramidal structure was on Earth, but wasn't made by humans. Each nation, I believe, sent a team of one or two people to compete in this odd treasure hunt for the keys. I'm not completely sure, but I think the reward for the winning team was that their nation was saved from the apocalypse? Something like that maybe, but now I'm not so sure that was the reward. But they were definitely competing for something very important, and something bad would happen to the losers.
The competitors could, and sometimes did, kill one another. Also the keys were spread out across the world. I think one of them was hidden at (or involved) Stonehenge, maybe? I also seem to remember that there were clues involved (again, kind of like a scavenger hunt). As far as I can recall, the aliens were never named in the book and never engaged in any kind of war with Earth. I remember the novel was more marketed towards people in their late teens, or else young adults. It was definitely published after the 80s, and was probably published sometime between 2002 and 2014.