The word forest in Scotland has a special meaning that confuses most visitors. A deer forest is not a wooded area; it is open hill country managed for the herding and stalking of red deer. Trees in such places were grazed out generations ago and have never recovered, kept short by the same deer the estates exist to harvest. Three estates cover the principal area: Dundonnell Estate (134 square km) in the north-west, including the northwestern flanks of An Teallach; the Eilean Darach estate (262 square km) covering the north-east; and the great Letterewe estate (323 square km) sprawling across the entire southern and central section. Together they make a roadless interior larger than the city of New York, owned in three parcels, traversed by no road and only a handful of footpaths.

Three of Scotland's most cherished mountains rise from this forest. An Teallach, southwest of Dundonnell, is a 1,062-metre Torridonian sandstone ridge whose name means The Forge - a reference, climbers say, to how the rock seems to glow in evening light. Its serrated skyline is one of the most photographed in the Highlands. A' Mhaighdean, rising northwest of Lochan Fada, is often called the remotest Munro in Britain: from any road in any direction, it is a long, hard walk in. Slioch, north of Loch Maree near Kinlochewe, is the great spear of rock that frames almost every photograph taken from the south side of the loch. A small bothy at Shenavall, in the floor of the wilderness between An Teallach and Beinn Dearg Mor, gives walkers a place to sleep out of the weather - one of the most isolated shelters of its kind in Britain.

The Fionn Loch is the largest body of water in the forest proper, its name meaning the white loch - white for the quartzite sand that lines its eastern shore, white for the pale northern light that hangs over it most of the year. Loch na Sealga, Lochan Fada and Loch a' Bhraoin scatter through the lower ground. The rivers - the Dundonnell, the Gruinard, the Little Gruinard, the Uisge Toll a' Mhadaidh - drain the wilderness in fast, peat-stained tumbles toward the sea. The salmon and sea trout in these waters made the estates valuable in the Victorian sporting era and continue to draw a small number of fishers each summer. Most days, on most of these lochs, you are likely to see no one at all.

The Great Wilderness owes its emptiness to a combination of geology, climate, and ownership. The bedrock is some of the oldest in Europe - Lewisian gneiss nearly three billion years old, overlaid in places with Torridonian sandstone half that age. The soils are thin and acidic. The weather sweeps off the Atlantic with little to soften it. Roads were never run through because no economic case existed: the population was tiny even before the Clearances of the 19th century, and what remained left for the coast. The private sporting estates kept the interior unbroken, even as forestry and infrastructure carved up wilder regions further south. Today the Letterewe Estate is associated with one of the more famous compromises in Scottish access law - the so-called Letterewe Accord of 1993 - which set out responsible stalker-walker coexistence and helped shape later national right-of-way legislation.

From the air, you can read the wilderness as a single pale-green expanse, slashed by silver lochs, with the An Teallach ridge running across the north like a line of broken teeth. There are no settlements to anchor the eye - only the bothy roof at Shenavall, glinting if the sun catches it. Pilots who fly the route from Inverness to Stornoway often note that, despite Britain's reputation as a crowded island, you can cross this stretch of Scotland and see no human structure at all between the A832 in the south and the A835 north of Ullapool. On a still day, the surface of Lochan Fada catches the clouds and turns the whole valley into a long mirror.