World's Most Secret Hotels

This episode explores Bolivia's Salar de Uyuni, a striking brutalist lodge seamlessly blending in with the edge of the world's largest salt flat. It also features a futuristic getaway of bubble domes in County Fermanagh in Northern Ireland, a mirrored resort overlooking a majestic Norwegian fjord, an art deco hotel nestles amid the Pyrenees, and the extra-terrestrial-inspired design of a unique homestay in Wales.

Visits to a fortress-style hotel in the Empty Quarter in the United Arab Emirates, a minimalist hotel on the tranquil banks of a Norwegian river, a futuristic fantasy with three bespoke treehouses in Dorset woodland, an eco-friendly yellow submarine on the river banks of County Monaghan in Ireland, and a luxury hotel fashioned from repurposed shipping containers in the New South Wales Australian outback.

An old Cornwall jail becomes an extraordinary luxury retreat, and opulent ancient caves bring the South African wilderness up close. A historic Swedish lighthouse on the rugged rocks of Paternoster Island shines a light for luxury seekers, while in tropical northern Queensland, an eco-resort offers stunning treetop accommodation.

Unusual destinations featured in this episode include a mountain retreat at the summit of Mount Pilatus in Switzerland, and a striking Brutalist hotel that transformed concrete into an architectural masterpiece. Off-grid in the English countryside, two handcrafted geodesic pods offer the ultimate digital detox, while a working lighthouse on La Palma secretly houses a luxurious hotel.

Featuring a South African nature reserve, Classic railway carriages repurposed as a luxury hotel in Kent, and curious submarine accommodation in the Scottish Highlands. An old freight crane is reborn as a harbour-side hotel in Hamburg, and there's a retro retreat in Utah, featuring wagons that never roll.

In the Bradshaw Mountains in Arizona, USA, hidden down an 11-kilometre dirt road, is a luxury hotel built around a geothermal wonder. At The Bus Stop in Scotland, there are nine luxury buses for hire, while a secret retreat in Poland has been designed to look like an open chest of drawers. Plus, a one-of-a-kind glamping pod in eastern England, and a boutique bolthole inside an ex-windmill in Suffolk.

In Cambodia we visit the Shinta Mani Wild hotel on the banks of the Timor Rung River, and in Italy, a clifftop settlement that's hewn into the limestone rocks.

We uncover more unique and unusual hotels in Jordan, England, Sweden and Ecuador, including the Bubble Wadi Rum Luxotel resort that resembles a colony on Mars and a converted plane.

We visit the TWA Hotel, hidden within one of New York's busiest airport runways, and Kakslauttanen Arctic Resort's glass igloos in Finnish Lapland.

More hidden hotels are revealed in Uruguay, Sweden, Scotland, Oman and The Netherlands - from a camouflaged resort perched on the edge of a mountain, to mainland Britain's most isolated hotel.

We travel to the Scottish wilderness in search of spaceships, the Welsh mountains to probe mysterious spheres, a Greek tower with a deadly history, and an Italian hotel perched on a glacier.

We uncover some truly remarkable and hidden retreats in Canada, Iceland, Sweden and the UK - from underwater sunken hotels to remote retreats surrounded by lava fields.