In the ‘Era of Martyrs’ at the end of the third century the persecuted Copts, the Christians of Egypt, suffered death or fled into the desert; but they found life possible only where there was water. The most favourable of these few oases was Nitria, today called ‘Wadi Natrun’ or Valley of Soda, a shallow depression of twenty-two miles by five in the Libyan desert surface. It is situated a camel journey of a day and a half along the ancient track which leaves the Delta near Terenthus, the modern El Tarrana. In it there are eight small lakes, some of whose waters contain soda, others salt. They lie about seventyfive feet below sea-level and are fed not by the Nile, but, as Dr Ball has recently shown, by desert subsoil water flowing from the southwest. Wells dug in their vicinity yield water, brackish but drinkable, and on the shore of one was the town of Nitriotis, the habitation of the sodaworkers and glass-blowers, existing since ancient Egyptian times, Now the desert is crossed by a light railway, run by a company that exploits the salt and sodium carbonate.