Wahiba Sands is 12,500 square kilometres of orange dune, and in the north its ridges run in long parallel lines up to 100 metres high. The colour comes from a film of iron oxide on the grains. Late in the day the sun rakes across them and every ripple throws a hard shadow, and from a crest the dune lines run in rows to the horizon. This is Bedouin country, not empty country. Al Wahiba families keep camels here and breed them to race, and from June to September they gather at the Al Huyawah oasis, which holds most of the sands' date palms, to bring in the harvest. Rain barely counts: under 50 millimetres a year. What keeps anything alive is sea air. Fog drifts in off the Arabian Sea, humidity at the sand surface climbs to 100 per cent after dark, and by morning plants are drinking the dew beaded on them.
Most people enter at Bidiyah, where drivers let the tyres down before the first ridge. Once you are over it the silence is broken only by sand hissing off the lip of the dune. The desert has no gate and no permit, but the camps along its northern edge mostly run from October to April and close when the summer wind and 40-degree heat arrive. Among the quartz grains underfoot lie shell fragments and marine carbonate: this sand blew inland off a continental shelf left bare when ice-age seas stood low, and the monsoon wind piled it into ridges. The Royal Geographical Society camped here from 1985 to 1987 to work out how the whole machine runs, logging some 150 plant and 200 animal species. At night the camp lights stop a few metres out, and past them the dunes hold nothing at all.