From the same as bathus; profundity, i.e. (by implication) extent; (figuratively) mystery -- deep(-ness, things), depth.
see GREEK bathus
1. properly: Matthew 13:5; Mark 4:5; Romans 8:39 (opposed to ὕψωμα); Ephesians 3:18 (opposed to ὕψος); of 'the deep' sea (the 'high seas'), Luke 5:4
2. metaphorically: ἡ κατά βάθους πτωχεία αὐτῶν, deep, extreme, poverty, 2 Corinthians 8:2; τά βάθη τοῦ Θεοῦ the deep things of God, things hidden and above man's scrutiny, especially the divine counsels, 1 Corinthians 2:10 (τοῦ Σατανᾶ, Revelation 2:24 Rec.; καρδίας ἀνθρώπου, Judith 8:14; (τά βαθα τῆς θείας γνώσεως, Clement of Rome, 1 Cor. 40, 1 [ET] (cf. Lightfoot at the passage))); inexhaustible abundance, immense amount, πλούτου, Romans 11:33 (so also Sophocles Aj. 130; βαθύς πλοῦτος, Aelian v. h. 3, 18; κακῶν (Aeschylus Pers. 465, 712); Euripides, Hel. 303; the Sept. Proverbs 18:3).