In this activity, learners create a map of a hidden surface using a "sounding stick" -- a technique similar to how underwater maps were once made. The hidden surface is created using plastic tubing inside of a Teaching Tank (a very narrow tank that is commercially available), and the sides of the tank are covered. A "sounding stick" made from either a dowel or drinking straw is used to measure the depth at 2" increments, and the points are graphed to create a profile map. Learners repeat the activity and reduce the increment (increasing the number of samples) to create a better resolution. Modern echo-based devices like sonar have made this technique less necessary, but it's still useful for understanding sampling and imaging.