So much for the various statements as to whereabouts Zerzura _may be_. Let us now inquire where it is _not_. On the map I have indicated the principal routes followed by explorers of the southern and western parts of the Libyan Desert during the last fifty years. We may be tolerably certain that Zerzura is not on, or very close to, any of these routes; for although a depression within a kilometre or so of one’s track may easily be passed without notice, the existence of a large oasis, such as most of the traditions make Zerzura out to be, would almost certainly be betrayed to an explorer by animal-tracks leading to it from considerable distances, except possibly in places where the ground was very sandy. [Illustration: _Pottery Hill from the south: at foot, petrol and water supplies of Prince Kemal el Din’s Expedition of 1923_] [Illustration: _Jars at northern foot of Pottery Hill found by Prince Kemal el Din in 1923_] [Illustration: _Jars, worn away by sand-laden winds, found by Dr. Ball in 1917 at southern foot of Pottery Hill_] There is another method now available to us by which we may narrow down the search for Zerzura or other “lost” oases; and that is, by a consideration of the general contours of the country and those of the static water-surface. Every oasis in the Libyan Desert must owe its existence to one or other of two conditions: either it must depend on springs fed by local rainfall, in which case, like the oases of Owenat and Arkenu, it probably lies near to mountains of considerable height; or else it must depend on underground supplies, and must therefore lie in a depression wherein the ground-level and static water-level are practically coincident, as in the cases of Kharga, Dakhla, Farafra, Baharia, and Siwa. It is highly doubtful whether any mountain masses at all approaching the altitude of Arkenu and Owenat can remain undiscovered in any of the various areas in which Zerzura has been traditionally placed. Zerzura is immensely more likely to be in a depression, and indeed Wilkinson’s name _Wadi_ Zerzura almost conclusively points to that view. The depression is more likely to be shallow than very deep, for it presumably lies in the sandstone country of the south-west, and all the known depressions of great depth are confined to the limestone country of the north-east. It is quite easy to trace out on our new map the areas wherein the static water-surface would be reached by a depression of say 50 or 100 metres below the general ground-level as indicated by the contours. We must, however, remember that our ground-contours are liable to be considerably in error in areas where observations have been few—that is, in precisely those areas where the depression, if it exists, is most likely to be found. So we must allow a good margin for our depth, and I have chosen 100 metres on this account. On the map I have drawn the “locus” of points in the southern part of the Libyan Desert where the static water-surface is 100