Thayer's Greek Lexicon has the following on καρδία

3.) used of the middle or central or inmost part of anything, even though inanimate: τῆς γῆς (which some understand of Hades, others of the sepulchre), Matthew 12:40 (τῆς θαλάσσης, Jonah 2:4 for לֵב; and for the same ἐν μέσῳ θαλάσσης, Exodus 15:8, 19; add Baruch 6: (Epistle Jer.) Baruch 6:19; τῆς κλεψυδρας, Aristotle, probl. 16, 8 (others, κώδια)). Cf. Beck, Biblical Seelenlehre, chapter iii. § 20ff, p. 64ff; Delitzsch, Biblical Psychologie (Leipz. 1861) iv. § 12, p. 248ff (also in Herzog 2, vi. 57ff); Oehler in Herzog vi., p. 15ff (also in his O. T. Theol. (edited by Day) § 71); Wittichen in Schenkel iii. 71f.

This doesn't preclude a symbolical or typological interpretation here, but regardless, the reference seems to be Hades (or Sheol in Jewish cosmology).