Once among Africa's largest lakes, it has shrunk dramatically since the 1960s; by widely published measurements its surface area has fallen to a small fraction of its former extent. Researchers cite changes in rainfall, upstream irrigation withdrawals and regional climate variation.

The lake sustains tens of millions across four countries at once: Chad, Niger, Nigeria and Cameroon. As the water recedes people move, often across borders because the lake sits where four countries meet. In the new country they are undocumented foreigners, and their children cannot be registered because the parents lack valid documents — exactly the circle recorded in Kenya, Nepal, Zimbabwe and Mauritania.

No. No international convention recognises displacement by environment as a protected legal status; the Refugee Convention rests on persecution, not receding water. There is a growing group for whom the international legal system has no box.

Because all nineteen arose from human decisions and could therefore be reversed by other decisions, as Madagascar did by amending one word. Here nobody decided anything — the lake simply got smaller — so there is no statute to amend, only the requirement that the countries around the lake agree among themselves.

You are buying the ability to move lawfully when circumstances change. Tens of millions around Lake Chad are having to move because circumstances changed, and they cannot buy that. Knowing it does not make your purchase wrong, but it tells you precisely what you are buying.

Via a pipeline to a port in Cameroon, because Chad is landlocked. It is the sixth landlocked variant recorded here, in the sharpest group with South Sudan: depending on a neighbour to sell what generates most of the budget.

The pipeline project was once tied to an international mechanism requiring a share of revenue to be spent on health, education and development, but it was not sustained. It was among the few times the world tried imposing a withdrawal rule on an oil country from outside, and that it did not hold is important for every oil-fund article: withdrawal rules hold only when they belong to the country itself, not when they are a lender's condition.

The Ennedi massif in the north with its rock arches and ancient rock art is a UNESCO site, as are the Ounianga lakes, a chain of lakes within the Sahara.