Re: GPS underwater?
You're good. There are two islands in the caldera, all formed by resurgent vulcanism since the caldera was formed. They're Nea Kameni, Palea Kameni which translate as New Burnt and Old Burnt respectively. They've been intermittently active for about 2000 years with the last explosive eruption in 1939-40 followed by quiet dome building in 1950 (IIRC).
This sort of deformation isn't uncommon in calderas as magma is regularly injected into the underlying magma chamber and then withdrawn again. It may, or may not presage more vulcanism in the very near future. If you want a good example of one that scared people witless a while back, the small town of Pozzuoli west of Naples sits in the middle of the Campi Flegrei caldera which last had a minor eruption in 1538 when the completely new volcano Monte Nuovo popped up in the middle of some fields. Between 1982 and 1984 they experienced thousands of small quakes and the ground bulging by 40cm - at its peak the town was rising by 4mm per day! Unsurprisingly, there were predictions of an imminent eruption, so the town was evacuated. After a few tense months the ground began to deflate and the area returned to its usual levels of seismicity.
And Campi Flegrei is a real monster of a volcano, about 37kya it produced 200km3 of white hot foam that now underlies most of the Bay of Naples. About 2 million people live in and around it who might need to be evacuated in a hurry. And you've seen what Italian traffic is like on a good day.