The bowels of the earth
The dark interior of the earth.
The simile that associates the bowels with the dark visceral centre of things is of long standing. The first known use of it is in Peter Morwyng's translation of The treasure of Evonymus, 1559:
"Sum put to it wormes or bowels of the earth."
Shakespeare was very fond of the imagery of 'the bowels of' and used it in several of his plays. In King Henry IV, Part I, 1596, he has Hotspur refer to 'the bowels of the harmless earth'. In Richard III, 1594, he used 'the bowels of the land'. In other plays he referred to 'the bowels of' - 'the battle', 'the commonwealth', 'the French', 'thy sovereign's son', 'the Lord' and 'ungrateful Rome'.
See other phrases and sayings from Shakespeare.