A Secchi disk is a simple tool used to measure water clarity in lakes and oceans. It consists of a circular disk, usually painted black and white, that is lowered into the water. The depth at which the disk becomes invisible to the observer indicates the water's transparency. This measurement helps scientists assess the health of aquatic ecosystems. Clear water typically supports more life, while murky water can indicate pollution or excessive algae growth. The Secchi disk is an essential instrument in environmental monitoring and research.