Water. We drink it, we bathe in it, we cook with it, we water our lawns with it, we go to water parks where pools are filled with it and streams of it come pouring down giant slides. We literally cannot live without it. We evolved from it and mostly consist of it.
And there are places on Earth which are about to run out of it. South Africa is probably the most urgent, but plenty more will follow.
The problem, of course, is that a lot of the world’s water is salt water, and we can’t drink salt water. Can’t water crops with it, either. Desalination would solve the problem, and we have the technology. It’s expensive, some might object, but most of that expense is because the process is energy intensive (there are other methods, but one is just to boil the water, run the steam through some pipes to cool it down, and collect the fresh water. It’s a large version of a still, or a mini-version of a rainstorm.
The thing is, energy is only artificially expensive. People make money off coal and oil burning power plants (and nuclear is kind of expensive, too, because of all the safety problems), but cheap energy (solar panels and windmills) is within our grasp and that would make desalination cheap and we could have fresh water anywhere in the world whenever we needed, or even wanted it.
It is an insane, and morally outrageous, state of affairs. People are going to die, possibly millions of people, because there is no money to be made in keeping them alive.