Although Venus is our closest neighbor, thick clouds hide its surface from even the most powerful telescopes. So in 1978, when astronomers observed a mysterious brightening in the planet’s northern hemisphere, they had no way to identify the cause. Toward the end of that year, though, the Pioneer Venus probe entered Venusian orbit--and immediately recorded suspiciously high levels of sulfur dioxide above the clouds. Larry Esposito, an astronomer at the University of Colorado, later proposed that a huge volcanic eruption on Venus in 1978 could explain both the brightening and the sulfur dioxide, a gas produced in eruptions on Earth. Last March, Esposito reported further support for his theory: the Hubble Space Telescope, making the first measurements of sulfur dioxide levels since the Pioneer mission ended in 1992, found they were the lowest ever recorded. They have been declining steadily ever since 1978--which is just what you’d expect if the sulfur dioxide came from an eruption in that year. Other researchers have fingered the giant volcano Maat Mons, shown here in an image made from radar data collected by the Magellan probe. But Esposito cautions, There’s still no smoking gun.