The desert is probably the greatest place on Earth because of how void of human presence it is and this is what we needed in order to create our underground labs. We have located many of them in some of the largest deserts in the world. The best thing is that there is no activity at all going on in the desert because we access all of the underground facilities from locations in the cities that then transport us to the underground facilities in the desert.
This is the main reason why we have such a great cover-up because no one has ever seen us entering into our facilities. The entry points are extremely well placed and no one would ever suspect them. The desert is our favorite kind of terrain for that reason and it will remain the center of our underground operations for a long time to come.