Best way to measure liquid level of a 10 meter high tank?
I don't know what is the “best”, and in fact I would be interested in
knowing the answer. So far the best option I found seems to be a
submersible level sensor (do a Web search for these words). These are
pressure sensors that you put at or near the bottom of the tank. They
give you a reading proportional to the height of water above the level
of the sensor.

Most of these sensors seems to give an output in the form of a
4&nbsp;–&nbsp;20&nbsp;mA current. They are have only two wires and
expect a supply somewhere in the range of 24&nbsp;V. You will then need
a suitable supply and some electronics to convert that to an analog
voltage that your Arduino can read. A simple resistor would be enough
but you probably want something that provides some safety in case the
sensor fails short. You can search this site for "4-20 mA", as the topic
of interfacing these kind of sensors has already been covered here
multiple times.