Etna. They have discovered that there is a time lag of 25 years between events. Monitoring of deep crust events can help predict accurately what will happen in the years to come. So far they have predicted that between 2007 and 2015, volcanic activity will be half of what it was in 1972. Other methods of predicting volcanic activity is by examining the increase of CO2/SO2 ratios. These ratios will indicate pre-eruptive degassing of magma chambers. A team of researchers used Mount Etna for this research by observing gases such as H2O, CO2, and SO2. The team did a real-time monitoring of Mount Etna before it experienced eruptions in July and December 2006. These CO2/SO2 ratios are useful in that the increase of these ratios are a predecessor to upcoming eruptions due to the acceleration of gas-rich magmas and replenishes the magma chamber. In the two years of observations that the team conducted, the increase of these ratios are a precursor to upcoming eruptions. It was recorded that in the months prior before an eruption, the ratios increased and led to an eruption after it had reached its peak amount. It was concluded that measuring H2O, CO2, and SO2 can be a useful method to predict volcanic activity, especially at Mount Etna. Mount Etna's prediction of volcanic activity can also be used with 4D microgravity analysis. This type of analysis uses GPS and synthetic aperture radar interferometry (InSAR). It can measure the changes in density, and afterwards, can retrieve a model to show magma movements and spatial scales that are occurring within a volcanic system. Back in 2001, gravity models detected that there was a decrease in the mass of Mount Etna of 2.5×1011 kg. Eventually, there was a sudden increase in the mass two weeks prior to an eruption. The volcano made up for this decrease in magma by retrieving more magma from its storage zone to bring up to the upper levels of the plumbing system. Due to this retrieval, it led to an eruption. The microgravity studies that were performed by this team shows the migration of magma and gas within a magma chamber prior to any eruption, which can be a useful method to any prediction of volcanic activity. Sakurajima, Japan Sakurajima is possibly one of the most monitored areas on earth. The Sakurajima Volcano lies near Kagoshima City, which has a population of over 500,000