The Thumbs are a group of distinctive rock formations on the east coast of the Tasman Peninsula, named for their shape above the water, with steep walls dropping into deep water below. Go Dive Tasmania lists them at 15-40 m or more, with vertical walls and sponge gardens, suitable for Advanced or Deep divers. The Marine Life Network guide places the Thumbs north of Fortescue Bay and describes a sharp wall drop-off in 30-35 m with seaweed on the upper sections and sea whips deeper down, along with butterfly perch and real bastard trumpeter.

The Divers Alert Network's Tasmania feature groups the Sisters and the Thumbs together as the advanced dives of the area, around 40 m deep and notable for sponge gardens, surge and currents that are impossible to predict. The top of the formation is a kelp-covered platform where fur seals may appear; the real interest is on the wall, where invertebrate cover becomes progressively richer below 20 m. Sponges in yellow, orange and red, fields of sea whips and gorgonians, and the endless schools of butterfly perch that characterise this coast make it a striking dive in good visibility.

It is a deep, exposed site for experienced divers only, and because depth limits bottom time it is typically a single focused descent rather than an exploration. The site is a good choice for divers who have done the Waterfall Bay caves and want the deep-water gardens without the long run to the Hippolytes. Operators will only go in settled conditions; be prepared for last-minute changes.

Boat access only. Eaglehawk Dive Centre and Go Dive Tasmania run from the Pirates Bay jetty at Eaglehawk Neck (about 75 km from Hobart); the Thumbs lie south along the coast past Waterfall Bay and O'Hara Bluff, roughly 20-25 minutes by boat. Private boats can also launch at the Fortescue Bay boat ramp (reached by the 12 km gravel Fortescue Road off the Arthur Highway near Taranna; national park pass required), which is a shorter run north to the site.

The Tasman Peninsula faces the full Southern Ocean and Tasman Sea. South-westerly to south-easterly groundswell and strong easterlies are what cancel trips: operators generally need westerly or north-westerly winds with swell under about 1.5-2 m for the open-coast sites. Prolonged southerly swell pushes surge deep into the caves and gutters and churns visibility; after a big blow allow two or three days to settle.

Late summer and autumn (February-May) bring the most settled windows and the clearest water; winter can be glassy between fronts but fronts arrive every few days. The Thumbs face east and are closed by easterly winds and swell; currents described as surge-prone and hard to predict.

Drop onto the top of the formation at about 15 m, move to the wall and descend to 30-35 m, then work along the drop-off through the sea whips. Begin the ascent early, spiralling back up the rock and doing a long safety stop on the shallow top.

Deep Specialty required by operators for the full profile; nitrox helpful for 30 m dives.

Go Dive Tasmania – The Thumbs (15-40 m+, Advanced or Deep) on Tasman Peninsula boat days; Eaglehawk Dive Centre – deep sponge-garden site in calm weather.