The Blue Grotto's light does not come in through the cave mouth — it comes up from underneath. Sunlight passes down through water clear enough to see the bottom, strikes a seabed of pale limestone sand, and bounces back up, so the sea itself becomes the lamp and the vaulted rock overhead glows a hard electric blue. Seven of these caves are strung along Malta's south coast, cut into Lower Coralline Limestone that was laid down around 25 million years ago, when the islands lay under a warm sea. The largest opening is a free-standing arch roughly 30 metres high, wide enough to take a boat, and it is the arch, not the colour, that Maltese speakers named the place after: Taħt il-Ħnejja, under the arch. Mineral streaks in the walls flash orange, green and purple as you pass, and those are the source of the old story that glowing algae light the caves — the colour is physics, not biology.
Boats leave from the slipway at Wied iż-Żurrieq, a fishing cove wedged into the cliffs below the road. Eight or ten passengers sit low in an open wooden boat; inside the caves the skipper cuts the motor, and nothing is left but water slapping stone and light moving across the ceiling. The window is narrow — by mid-afternoon the cliff throws the openings into shadow and the blue drains out of the water. Boats sail all year whenever the sea allows, but from November to March northerly winds shut them down for days at a stretch, and a rough morning cancels the trip on the spot. Coming anyway costs you nothing: a free lookout on the road above the harbour frames the arch, the boats crawling beneath it, and the uninhabited islet of Filfla out on the horizon.