In some places, especially some new wells that have just been drilled,
the oil is under pressure, and it brings itself to the surface as soon as
you drill a 'pipe' for it to rise through.
In most oil wells, there's a pump bobbing up and down day and night,
pumping the oil up out of the well.
When the well is so old that even a pump isn't very effective, water is
often forced down the well under pressure, and the water forces the
oil back up through the pipe.
