A propagating medium may be teeming with a multitude of weak signals even when it appears superficially to be empty. For example, an acoustic medium such as a lake may appear quite still, and yet it may contain numerous faint pressure waves originating in various submerged objects and reflecting off other submerged objects. A passive sonar system can intercept these waves and extract useful information from the raw received data. Indeed, these invisible pressure waves can be used in principle to form images of submerged objects. Likewise, a seismographic sensor or an array of such sensors on the surface of the earth can measure tiny vibrations of the earth's surface and deduce the location of distant earthquakes and other geophysical disruptions or can form images of geological structures. Even the electromagnetic environment in which we are immersed contains immense quantities of information. For example, some of these electromagnetic signals can be intercepted by suitable large apertures and formed into detailed images of far distant galaxies. We need provide no illumination, nor can we provide illumination in such an application. We need only gather the data with appropriate passive sensors and process the sensed data into the images that allow us to observe these galaxies.