I read somewhere, too lazy to google, that some of these super-deep earth bacteria live a sort of slowed-down life, with decades and longer going by between fission events. It's pretty cool to imagine this parallel unseen biosphere slowly ticking away beneath our feet.
@3 I heard (on NPR I think) that one species of bacteria in deep ocean sediment divides every thousand years! I can't find the NPR link, but something like this:
This is why I think we'll find microbes on (under) Mars, Europa, etc. Bacteria need almost nothing to live on: a little bit of water, and some source of metal or bubbling gas that's an electron acceptor or donor. Pretty much any energy source can be exploited; sunlight is not really necessary, it seems.
He also thought oil is abiotic...as supported by increasing deep well drilling.
You were talking about living in the cracks.
http://www.msnbc.msn.com/id/47465770/ns/…