Galileo spacecraft reveals an ocean of magma

Question: What is the Solar System's most volcanic body? Give up? NASA's Galileo spacecraft has returned data indicating a subsurface ocean of liquid or partially liquid magma on Jupiter's moon Io. This molten layer goes a long way towards explaining why Io is the most volcanic body in the Solar System.

From the NASA article:

• Io produces about 100 times more lava each year than all the volcanoes on Earth. While Earth's volcanoes occur in localized hotspots like the "Ring of Fire" around the Pacific Ocean, Io's volcanoes are distributed all over its surface. A global magma ocean about 20 to 30 miles beneath Io's crust

The NASA article really goes into detail about how Io has been studied and how the volcanic activity was discovered. Its a very informative article.