The visual cliff experiment is an experiment originally designed to to determine if an infant has developed depth perception. The visual cliff was created by covering a drop from one surface to another with see-through glass. In the original study (by E.J. Gibson and R.D. Walk), the majority of infants who had begun to crawl refused to venture onto the seemingly unsupported surface, even when their mothers beckoned encouragingly from the other side.
Gibson, E. J., & Walk, R. D. (1960). The “visual cliff.” Scientific American, 202 (4), 67-71.