Solution to 6: Aegean Treasure

The flavour text clues ancient Greece, squares, and scales. To decode the message, use the ancient Polybius square on the Greek alphabet it was originally designed for:

11 24 35 31 24 43 -> alpha iota omicron lambda iota sigma = Aeolis
14 54 42 24 43 -> delta omega rho iota sigma = Doris
24 54 33 24 11 -> iota omega nu iota alpha = Ionia
31 35 25 42 24 43 -> lambda omicron kappa rho iota sigma = Locris
31 45 14 24 11 -> lambda upsilon delta iota alpha = Lydia
51 42 45 13 24 11 -> phi rho upsilon gamma iota alpha = Phrygia

Which are regions of the ancient Greek world, mostly in Anatolia (Modern-day Turkey). They correspond to musical modes derived from the major scale (Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, Locrian). The answer is the missing Mixolydian, which means "half-Lydian" and is not a location.

MIXOLYDIAN