Jupiter's moon Io is a hotbed of geological activity, with over 400 active volcanoes. Because of this high level of activity, scientists have been debating for years about what's going on under the moon's surface. Now we know.
Below a low-density crust of about 30 km lies a seething ocean of magma at least 50 km deep (about 31 miles), which explains the constant volcanic explosions that make this moon the most geologically active body in the solar system.
As a point of comparison, the deepest part of Earth's oceans is 6.77 miles, at the base of the Mariana Trench.
Researchers at UCLA and UC Santa Cruz puzzled this out using existing data from the Galileo probe. Recent discoveries in the world of minerals have revealed that certain types of volcanic rocks can carry an incredible electrical current when molten. Looking at the shape of Jupiter's magnetic field gathered from Galileo, they surmised that warps in that field must have been caused by an enormous magma ocean beneath Io's surface.
Short of sending a probe to the moon's surface, this is the closest glimpse we'll get at this distant and explosive moon.