Vesuvius (Vesuvio) forms a dramatic backdrop to the Bay of Naples. The historically active cone of Vesuvius was constructed within a large caldera of the ancestral Monte Somma volcano, thought to have formed incrementally beginning about 17,000 years ago. The Monte Somma caldera wall has channeled lava flows and pyroclastic flows primarily to the south and west. Eight major explosive eruptions have taken place in the last 17,000 years, often accompanied by large pyroclastic flows and surges, such as during the well-known 79 AD Pompeii eruption. Intermittent eruptions since 79 AD were followed by a period of frequent long-term explosive and effusive eruptions beginning in 1631 and lasting until 1944. The 1631 eruption was the largest since 79 AD and produced devastating pyroclastic flows. Many towns are located on the volcano's flanks, and several million people live within areas potentially affected by eruptions of Vesuvius. (Global Volcanism Program)