One such supervolcano, at Yellowstone national park in the US, is overdue for an eruption. Yellowstone has gone off roughly once every 600,000 years, the last eruption about 640,000 years ago. The Yellowstone caldera is 70 kilometers long and 30 km wide, with a huge magma chamber 8 km beneath the Earth's surface. Ground surface deformation and other signs measured by satellite suggest it's still active, and on the move.
Another caldera is in the Phlegraean Fields near Naples in southern Italy. It is similar to Yellowstone, but smaller.
Dus toch in de buurt van de Vesuvius...