A sounding pole is a long rod, usually wood or aluminum, that is used to measure depths. They are primarily used to measure bodies of water, but can be used to measure the depths of holes, snow, and soil deposits in certain areas.
Generally, a sounding pole is about 20 feet in length, marked off in one inch increments. They tend to have a circular plate at one end to ensure that the spot being measured is fairly flat and normalized. The operator simply drops the pole until there is a refusal to go further, and then a measurement is taken. Usually this is done two or three times in close proximity to get a consistent reading.
Sounding poles have been replaced by SONAR and other bathymetric devices, but can still be used in areas where large amounts of vegetation or viscous mud distort electronic responses.
Fun Fact: Samuel Clemens got his pen name, "Mark Twain", from his time working as a leadsman on the Mississippi River. The leadsman was the person who used the sounding pole or lead line to measure the depth of the river to ensure the boat was staying away from the shallow banks. To "mark twain" meant to mark that the depth of the water was two fathoms, or 12 feet.