The Secchi disk, created in 1865 by the Italian Pietro Angelo Secchi, was originally made from a dinner plate.
Slightly more refined versions made from plastic or metal are used to measure water transparency in lakes, rivers, estuaries and the oceans. The disk is lowered through the water column until it can no longer be seen, raised until it reappears, and the average of those two depths is the "Secchi depth". That depth can range from a few centimeters in very turbid or algal-laden water, to tens of meters in oligotrophic lakes like Tahoe or Crater Lake, and especially in the open ocean.
This photo shows an improvised Secchi "plate" made after a student continued to lower his real disk beyond the length of the rope (10 m), and then watched as the disk and rope disappeared into the depths of Lake Powell (Utah-Arizona, USA).