So the power is out, and you need to retrieve some precious liquid from underground. It doesn't matter if bad guys have attacked, or a storm blew over a bunch of trees - when you flip the switch on the pump, nothing happens.
The liquid you need could be water in the bottom of the well, or gasoline in an underground tank.
Turns out all you need is an airtight chamber whose volume can be changed at will, and two, one-way valves, and some hoses.
It can be as simple as a soda bottle and two pieces of tape:http://youtu.be/ZcyyMPJQ36Y
or a set of PVC pipes with some one-way ball valves made with marbles:http://youtu.be/x5m9Ue_YqVs
Granted, the first would need a tube, and would become less and less effective with height (QUICKLY!) but it demonstrates the principles you need to know to give you a basic understanding.
Having a basic understanding of how they work will allow you to improvise one from found materials, retrieve your objective, and move on to bigger problems.