This would be one of the most sensible alien theories I have come across:

Professor Paul Davies, from the Australian Centre for Astrobiology at Macquarie University in Sydney, believes a cosmic greeting card could have been left in every human cell. The coded message would only be discovered once the human race had the technology to read and understand it. td>

It's the same as I suspect for the Nazca Lines - only people who can fly will see them, and perhaps people who fly are also smart enough to decode them...

Source: Mail & Guardian