Spoiler Alert

Mr Stephenson explicitly says that the Pingers derive from a secret (US Navy?) project to allow submariners like Cal to survive underwater.  Given that all resources of the global governments were supposed to have been devoted to sending people and objects into orbit, it's hard to believe that this underwater project could have been pulled off without Julia being informed about it.  This is in contrast to Rufus Macquarie's project, which was private and could well have been under the radar of the US Government.  Although this will likely never be explained or explored, even in the event that Mr Stephenson decides to write a sequel (which I'd love, by the way!) I would be very interested to know whether Julia did know about the project, and more importantly whether she ever passed on knowledge of the possibility to her children.