A tide gauge is a device for measuring sea level and detecting tsunamis.

Sensors continuously record the height of the water level with respect to a height reference surface close to the geoid. Water enters the device by the bottom pipe  and electronic sensors measure its height and record it to a tiny computer.

Data is available for over 1,750 stations worldwide. At some places records cover centuries, for example in Amsterdam where data dating back to 1700 is available. When it comes to estimating the greater ocean picture, new modern tide gauges can often be improved upon by using satellite data.

Tide gauges are used to measure tides and quantify the size of tsunamis. The measurements make it possible to derive the mean sea level. Using this method, sea level slopes up to several 0.1 m/1000 km and more have been detected.

A tsunami can be detected when the sea level begins to rise, although warnings from seismic activity can be more useful.

http://en.wikipedia.org/wiki/Tide_gauge