  Muon imaging has recently emerged as a powerful method to complement standard
geophysical tools in the understanding of the Earth's subsurface. Muon
measurements can yield a radiography of the average density along the muon
path, allowing to image large volumes of a geological body from a single
observation point. Here we jointly invert muon data from three simultaneous
telescope acquisitions together with gravity data to estimate the
three-dimensional density structure of the La Soufri\`ere de Guadeloupe lava
Dome. Our unique dataset allows us to achieve an unprecedented spatial
resolution with this novel technique. The retrieved density model reveals an
extensive, low-density anomaly where the most active part of the volcanic
hydrothermal system is located, supporting previous studies that indicate this
region as the most likely to be involved in a partial edifice collapse.
