A mooring is an instrument, or series of instruments, attached along a cable that extends over a portion of the water column. A flotation buoy at the top of the mooring keeps the cable taught, and a heavy anchor at the bottom keeps the mooring from moving around. A variety of instruments can be attached to the mooring line; for instance, current meters (which measure the water flow- how fast is it moving?) and temperature/salinity sensors. These instruments collect data at specific time intervals and store the data until the mooring is recovered at a later date.