The Blue Holes of the Vernon Islands are one of the Top End's natural curiosities. Scattered across the reef flats – large ones and small ones, with the biggest on North West Vernon Island – they are holes in the limestone-rich reef platform, thought to have formed by rain erosion and chemical weathering of the reef during past ice ages when sea levels were lower. At low tide the surrounding reef and sand banks are exposed and each hole becomes a landlocked pool; at high tide the flats flood again.

Because they are cut off from the current at low water, the holes offer reasonably sheltered snorkelling – a rare thing in Darwin waters. The largest hole has tall vase and staghorn corals on its walls and schools of brightly coloured fish. Visitors have described yellowtail fusiliers, golden trevally, angelfish, cardinalfish, parrotfish and batfish, a hawksbill turtle, juvenile blacktip reef sharks and tawny nurse sharks inside the pools, and a striking 'horizontal waterfall' where water drains out of the hole over a limestone lip as the tide falls. The water is warm and, when conditions are right, clear.

Around the holes, the exposed reef flat is worth a careful walk for its invertebrates: sea turtles, blue-ringed octopus, mantis shrimps, ass's-ear abalone and many snails and crabs have been recorded.

This is a snorkelling and reef-walking site rather than a scuba dive, and it depends entirely on tide timing. It suits families and naturalists with access to a boat and a local skipper who knows the reef, as well as divers looking for a relaxed low-tide swim between drift dives on the Vernon reef edges.

The Vernon Islands lie in Clarence Strait between Gunn Point and Melville Island. The closest launch is the concrete ramp at Leaders Creek, which the Marine Life Network notes is quite close to the islands (drive via the Arnhem Highway and Point Stuart/Leaders Creek roads, roughly 2.5 hours from Darwin). Boats from Darwin's Buffalo Creek ramp at Lee Point or from Cullen Bay run east across Shoal Bay and past Gunn Point – a longer open-water trip.

The islands are ringed by reef flats that dry at low tide; approach on a rising tide with a careful lookout.

The blue holes are a tide-dependent snorkel site. At high water the reef flat floods and boats can approach; as the tide falls the holes become landlocked pools protected from the current, which is when the Marine Life Network says snorkelling is reasonably sheltered. Plan the visit around a mid-to-low tide with enough water to get the boat off afterwards – visitors on a Sea Darwin trip had to leave within an hour when the tide turned.

Dry season calm mornings only; wet-season monsoon, cyclone risk and box jellyfish.

Anchor off the reef edge on a falling tide, walk or swim across the flat to the largest hole on North West Vernon, and snorkel slowly around its rim and walls. Keep an eye on the tide and return to the boat before the flat floods and the current runs.

Sea Darwin has run occasional Vernon Islands reef walk and snorkel trips (with the NT Field Naturalists and Australian Marine Science Association); otherwise private boats only.