Our group of divers drop in to the Tanon Strait and skirts the shallow coral reef, just a few metres below the surface. We fin slowly past vast beds of cabbage coral, the blue above the yellow coral is a flurry of small-fish life.
But that's not the main attraction. This strait, which runs between the islands of Negros to the west and Cebu to the east, in the centre of the Philippines, offers one of those rare National Geographic moments for divers, when the fish you've only seen on screen come to life suddenly.
These are Sardines. Millions of them, swimming in thick clouds around, above and below you, so thick you can hardly see at times.
They race towards us, each tiny silver shimmy an individual piece of an enormous, shifting 'metallic' school. It breaks this way and that, flowing up and down, groups dissolving one moment and re-shaping the next. The heart of the action is Pescador Island, an uninhabited rock outcrop just off the dive town of Moalboal. The island's edges have been eaten away by the sea, and its jagged walls drop 10 metres into the strait. The only structures are a few temporary shelters for fishermen caught in storms and a tiny white lighthouse that pokes above the green bushes clinging to the island top.
Pescador isn't much to look at - it's the waters around the island that are the attraction. A flotilla of fishermen in their one-man bancas patrol the area just offshore, the thin hulls of their narrow outriggers brilliant in teal, blue or red. Each wind-worn fisherman sinks a single line into the waters below, and sits and waits, a picture of patience. Below them, the hooks flash silver in the water, aiming to catch the eye of the passing fish.
The fishermen are after bigger prey - the mackerel, tuna, barracuda, grouper, jacks and snapper that follow the sardine schools.
There are even bigger species in the deep blue, which you may spot if you're lucky. A Dutch family diving with us is fortunate enough to come across a rare thresher shark and a whale shark, the largest fish in the sea, on the same dive.
The shark meanders off into the strait, leaving Pescador Island and its sardines safely behind.
We search for its huge bulk on a few subsequent dives, but come across only the silver clouds again and again.
A scattershow to rival any man-made fireworks, we watch the sardine flashes flare, fade and reform all around us, then head for shore.
The Philippine Siren, run by Worldwide Dive and Sail (www.worldwidediveandsail.com ), is a live-aboard vessel that visits the central Visayas, Ta?on Strait, and Pescador Island. A 10-day trip costs Euro2,400 (HK$26,088).
There is a wide selection of dive resorts in Moalboal, including Turtle Bay Dive Resort (www.turtlebaydiveresort.com ), which runs dive trips that seek out the sardines specifically.