The sea caves below Polignano a Mare were carved by rain, not by the Adriatic. Fresh water sinking through Cretaceous limestone dissolved the Murge plateau from the inside during the last glacial period, when the shoreline lay lower and further east. Rising meltwater later flooded chambers it had never cut. Polignano a Mare Boat Cruise to Scenic Caves with Prosecco tours therefore travel through drowned karst — dry passages that the sea inherited rather than built. The town above them is older than its cliffs suggest. Roman engineers pushed the Via Traiana through here in 109 AD, and the bridge that still spans the ravine at Lama Monachile follows that line. Fishermen worked the grottoes for centuries, naming them by shape and occupant: Grotta delle Rondinelle for its swallows, Grotta dei Colombi for its pigeons, Grotta Ardito for the family that owned the land above. Grotta Palazzese acquired a stranger function. In the eighteenth century local nobles held banquets inside its mouth, hanging lanterns from the limestone and dining above the swell. The practice was recorded by travellers on the Grand Tour; the cave now holds a dining room built into the same hollow. That layering explains why the grottoes matter beyond geology. They are the reason Polignano developed seaward rather than inland, its white houses set flush to a twenty-metre drop. Domenico Modugno, born in the town in 1928, wrote "Nel blu dipinto di blu" about a sky the colour of this water; his statue faces the same cliffs from the seafront. Divers launch from those ledges in summer competitions watched from the ravine. A giro in barca alle grotte remains the only way to read the coast from below, where the erosion is legible: notches at the old sea level, collapsed roofs open to daylight, arches thinning towards the point where they will fail. The chemistry is simple and slow. Rainwater takes carbon dioxide from air and soil, becomes weakly acidic, and widens every joint in the rock at a rate measured in centimetres per millennium. Waves then quarry what the acid loosened, working along fracture lines until a passage breaks through to open water. Grotta Palazzese, Grotta Ardito and the smaller hollows east of Cala Paura all sit on the same fault-guided grain. Light does the rest: the Adriatic here carries little sediment, so sunlight reaches the seabed and returns upward through the water column, filling low chambers with a green cast that shifts as the sun moves. Any Polignano a Mare Boat Cruise to Scenic Caves with Prosecco tour is, in effect, a transect through half a million years of dissolved limestone — a Puglia coastal landmark still, quietly, being unmade.