In the book Diaspora, ancient aliens replaced the elements with the unusual form - I forget the details, but say N15 instead of the common N14, O15 instead of O16. Gives a big bright signal that something unusual is happening on this planet.
In the book Diaspora, ancient aliens replaced the elements with the unusual form - I forget the details, but say N15 instead of the common N14, O15 instead of O16. Gives a big bright signal that something unusual is happening on this planet.