inner space

noun
  1. the environment beneath the surface of the sea
  2. the human mind regarded as being as unknown or as unfathomable as space
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012