Commenting on Psalm 6:5, Derek Kidner provides an excellent summary of Old Testament teaching on ‘sheol’ (the grave; the abode of the dead) and what lies beyond:-

Sheol can be pictured in a number of ways: chiefly as a vast sepulchral cavern (cf. Ezek. 32:18–32) or stronghold (Pss 9:13; 107:18; Matt. 16:18); but also as a dark wasteland (Job 10:22) or as a beast of prey (e.g. Isa. 5:14; Jon. 2:2; Hab. 2:5). This is not definitive language, but poetic and evocative; and it is matched by various phrases that highlight the tragedy of death as that which silences a man’s worship (as here; cf 30:9; 88:10f.;