The Arecibo Observatory was constructed in 1963 as the world’s largest and most sensitive radio telescope. Sitting in a naturally spherical valley in Puerto Rico, this telescope looks decidedly different from the pristine optics associated with optical telescopes such as the Keck Observatory or the Hubble Space Telescope. This is because the Arecibo Observatory peers into the Universe with radio waves. These radio waves have large wavelengths, between 3 cm and 1 meter, so the green vegetation stains on the bottom hardly make a difference. In fact, the radio dish isn’t even a smooth round shape; the spherical dish is comprised of 38,778 suspended flat steel plates that approximate the spherical shape used to concentrate radio signals to the moving receiver and transmitter suspended above the dish by three cables.
The unique transmitting feature of the Arecibo Observatory was put to use in 1974, when at the behest of Dr. Frank Drake (author of the Drake equation) and Carl Sagan, a binary message was sent towards the star cluster Messier 13. This message (more information here) contains information about life on Earth, human biology, our place in the solar system, and current Earth technologies. If we are lucky (or potentially unlucky), aliens should be able to pick up this signal from M13 within about 25,000 years, when the light speed signal reaches these distant stars. Now we Earthlings have to patiently wait around 50,000 years for a potential return signal, but in the meantime, the Arecibo Observatory has been busy making radar mappings of the surface of Venus, finding Soviet radar stations by observing radio waves reflected from the Moon, and generally advancing our knowledge of the Universe.