Microscopic organisms called animalcules were first observed under the microscope in the late 1600s by Antonie van Leeuwenhoek, who saw single-celled organisms moving in pond water. Leeuwenhoek's observations marked the discovery of microorganisms and helped establish microbiology as a field of scientific study. His findings challenged prevailing assumptions that life was visible to the naked eye and demonstrated the immense diversity of microscopic life that exists all around us.