Hongs are the pièce de résistance of Ao Phang Nga: invisible to any passing vessel, these secret tidal lagoons are enclosed within the core of seemingly impenetrable limestone outcrops, accessible via murky tunnels that can only be navigated at certain tides in kayaks small enough to slip beneath and between low-lying rocky overhangs. Like the karsts themselves, the hongs have taken millions of years to form, with the softer limestone hollowed out from above by the wind and the rain, and from the side by the pounding waves. Eventually, when the two hollows met, the heart of the karst was able to fill with water via the wave-eroded passageway at sea level, creating a lagoon. The world inside these roofless hollows is an extraordinary one, protected from the open bay by a ring of cliff faces hung with vertiginous prehistoric-looking gardens of upside-down cycads, twisted bonsai palms and tangled ferns. And as the tide withdraws, the hong’s resident creatures emerge to forage on the muddy floor, among them fiddler crabs, mudskippers, dusky langurs and crab-eating macaques, with white-bellied sea eagles often hovering overhead.