The Fossil Cliffs are tall limestone cliffs on the northern tip of Maria Island, famous on land for the dense beds of Permian fossils - mostly shells and bryozoans - that make up the rock. Under water, the Marine Life Network guide describes a sunken sea terrace near Cape Boullanger with a large swim-through, about 4 m wide, 5 m high and 20 m long, at around 17 m, with the sandy fringe as the landmark for finding it. It rates the site 6.5 out of 10.

Bullseyes gather in the swim-through and banded morwong patrol the terrace. Above it, at about 10 m, is a weed garden with typical east coast reef fish - wrasse, leatherjackets, sea sweep and trumpeter. The same fossil-bearing limestone forms the terrace, and fossil shells can be seen in the rock, although collecting is prohibited in both the national park and the marine reserve.

The neighbouring Fossil Bay, to the west, is a shallower site of 5-12 m, and the two can be combined. The swim-through is spacious and not a true cave, but divers should still avoid it in surge.

Fossil Cliffs suits Open Water divers comfortable with boat diving and a short overhead section. There is no dive operator on the island, so visitors need their own boat or a club trip from Triabunna.

Maria Island is a national park with no permanent residents apart from rangers, and the Darlington settlement preserves convict-era buildings from 1825 onwards. The marine reserve along the island's northern and western shores has been no-take since 1991 and has been used for long-term studies of how protection affects rock lobster and fish numbers, so reef life inside it is noticeably richer than outside.

Triabunna, on the Tasman Highway 1 hr 15 min (85 km) north-east of Hobart, is the gateway. The Maria Island passenger ferry (Encounter Maria Island) runs several times daily from Triabunna to the Darlington jetty, about 16 km and roughly 30-45 minutes; it carries people and small amounts of luggage but not vehicles, and dive cylinders should be cleared with the operator first. Private boats launch at the Triabunna ramp (best), Saltworks Road ramp, or Orford (shallow bar at the Prosser River mouth - difficult at low tide).

The Fossil Cliffs are at the northern tip of the island near Cape Boullanger, about 2 km from Darlington. By boat, the site is about 30-40 minutes from Triabunna across Mercury Passage. Walkers can reach the cliff top on the Fossil Cliffs circuit from Darlington, but the dive site is boat-only.

Maria Island's western and northern shores (Darlington, Fossil Cliffs, Painted Cliffs) are sheltered from easterly ocean swell but exposed to strong westerlies and north-westerlies blowing across Mercury Passage, which are common in winter and spring. The eastern side (Mistaken Cape, Bunker Bay) faces the open Tasman Sea and needs westerly weather and small swell. Southerly swell wraps around Cape Peron and the southern end.

Visibility drops after rain as runoff flows from the Prosser River into Mercury Passage. The northern tip catches northerly and north-easterly swell and wind; best in southerly or light westerly weather.

Drop near the sandy fringe at the base of the terrace at about 17 m, find the 4 x 5 m swim-through (about 20 m long) and pass through it, then rise to the weed garden on top of the terrace at about 10 m and finish along the base of the cliffs.

No operator based on Maria Island. Hobart shops run occasional trips: Go Dive Tasmania has offered Ile des Phoques seal dives and filmed Troy D dives; Tasmanian Scuba Diving Club and Tas Uni Dive Club run club trips from Triabunna. Most diving is from private boats.