Overview: Leeches - A century after falling out of favor, medicinal leeches are back in hospitals, sucking away on patients' wounds. SETI - Astronomers have their radio telescopes tuned to receive signals from alien worlds. But is anybody out there? Stem Cells Breakthrough - Three separate teams overcome a biomedical hurdle—creating stem cells without the use of human embryos. Profile: Edith Widder - Meet a marine biologist and explorer who has engineered new ways to spy on deep-sea creatures.