A.T. Robertson Commentary Luke 11:44

"Woe unto you! for ye are as the tombs which appear not, and the men that walk over [them] know it not." — Luke 11:44 (ASV)

The tombs which appear not (τα μνηνεια τα αδηλα). These hidden graves would give ceremonial defilement for seven days (Numbers 19:16). Hence they were usually whitewashed as a warning. So in Mt 23:27 the Pharisees are called "whited sepulchres." Men do not know how rotten they are. The word αδηλος (α privative and δηλος, apparent or plain) occurs in the N.T. only here and 1 Corinthians 14:8, though an old and common word.

Here men walking around (περιπατουντες) walk over the tombs without knowing it. These three woes cut to the quick and evidently made the Pharisees wince.