Quetzal, I've read two books on the matter: "The deep hot biosphere" by Thomas Gold, and "The fifth miracle" by Paul Davies. There are some Astrobiology web pages that mention the hypothsesis:
http://www.pathwaysforinquiry.com/studentpapers/wallacebigidept2.html
http://waynesword.palomar.edu/ploct97.htm
http://www.spacedaily.com/news/life-01zd.html
I have read somewhere that big sterilizing impacts could have lasted as late as 3,8 billions years ago. Any way, if by 3,5 billion years forms of life as complex as cyanobacteria (stromatolites) were already present, the time for life to evolve seems a little scarce. Here is where the possibility of life surviving underground fits nicely.