Metro 2033 (2005) and Metro 2034 (2009) by Dmitry Glukhovsky are post-nuclear-apocalyptic novels which describe the last remaining humans fighting to survive in the metro system underneath Moscow with the surface being too irradiated for humans to survive.
Against the Day (2006) by Thomas Pynchon makes extensive mention of the Earth's interior as a place to be explored, positing inner-Earth seas.