If you are a volcanologist, nothing strikes fear into your heart as much as thinking about the next Vesuvius eruption. This Italian giant is nestled in the sprawling metropolitan area of Naples, population 3.1 million. We're not talking "nearby" like Rainier is to Seattle or Popocatépetl to Mexico City. We're talking a volcano smack in the middle of the city. It is merely ~12 km (~7.5 miles) from the summit craters at Vesuvius to downtown Naples.

For your average pyroclastic flow from a volcano like Vesuvius, that is a trip that would take only about two and a half minutes. We've been lucky so far, as the growth of Naples since World War II has all occurred while Vesuvius has been quiet—its last eruption was in 1944—but planning needs to be done now for the eruption that will come in the future.

We can be sure that Vesuvius hasn't gone down for the count. Looking back over the last few thousand years, which for a volcano is a very short period of time, Vesuvius has had 42 eruptions that rank as VEI 3 or larger. On that Volcano Explosivity Index, VEI 3 means that over 10,000,000 cubic meters (2.9 billion gallons!) of volcanic ash and debris erupted.

That's not all. Vesuvius has gone even bigger. The eruption in 1631 was VEI 5, which is on the scale of the 1980 eruption of Mount St. Helens or 2011's Puyehue-Cordon Caulle eruption in Chile/Argentina. Both of those comparison eruptions happened in fairly unpopulated areas. But plop something that emits at least a cubic kilometer (over 350 billion cubic feet!) of volcanic ash and debris in the middle of an urban area ... it is really something we haven't seen in modern history.

This is why volcanic hazard planning is so vital to cities on volcanoes. Imagine a scenario where an earthquake swarm and small explosions start happening at Vesuvius, both signs that a larger eruption is in the works. How do you start deciding who to evacuate? An exclusion zone of 5 kilometers from the volcano means an evacuation of ~17,000 people. That is a lot but not wholly untenable.

Now, let's say the explosions start getting more and more vigorous: Vesuvius might be headed to a big eruption. Expand that exclusion zone to 10 kilometers from the crater and we are know looking at evacuating over 675,000 people. A really big eruption will blanket ash in an area tens to hundreds of kilometers from Vesuvius, impacting a population that is ~6 million people.

That would be 6 million people who could lose access to power, water and transportation if Vesuvius unleashes an eruption like was seen in 79 A.D. or 1631. How long would they have to stay away from their homes and livelihoods to be safe? As we've seen around volcanoes like Merapi and Sinabung in Indonesia, it could be years to ... forever. If parts of the Naples area are buried by volcanic material during an eruption, they could be uninhabitable for decades as that loose volcanic debris gets washed down the slopes of Vesuvius during every rainstorm, as like what happened around Pinatubo after its 1991 eruption.

So, much like Seattle and the Pacific Northwest need to prepare for the unthinkable earthquake, Naples and Italy need to prepare for the unthinkable eruption from Vesuvius. They have been getting ready with plans and research on how the region could be impacted, what is the likelihood of a massive eruption and how effective current mitigation plans might be.

However, it is communication of these risks that will make the difference. People need to know what different signs of volcanic unrest mean and in particular, what do geologists mean what we say that an eruption "may occur". Getting people on the ground ready through education, emergency kits, plans for evacuation routes and shelters is vitally important, but if the population doesn't understand how and when to evacuation, you end up with catastrophes like Armero or L'Aquila.

The thing is, comparing preparedness for earthquake and eruptions is difficult. Sure, they are broadly similar: have a plan, have supplies, be ready at a moment's notice. However, for an earthquake, you never know when the Big One might strike and likely, you will never know until it happens. Eruptions, on the other hand, have precursors, but those precursors are a bit of a mixed blessing. Sure, they let you know that a volcano is getting restless and an eruption might happen, but the devil is in the details.

Unrest might mean eruption, but it also might mean nothing but unrest. It also might mean months and months of unrest that lead to something small. In the realm of predicting a volcanic eruption, for every Pinatubo where volcanologists nailed it, there is a Long Valley where nothing happened. (Actually, these days volcanologists are much better, so maybe 3 Pinatubos for every Long Valley). It's like living waiting for a hurricane when it gets windy and rainy, but you're not sure if it will make landfall tomorrow or in 6 months.

The question of when to evacuate is one of the biggest challenges for volcanic disaster mitigation. Call for an evacuation too soon and nothing happens, you lose the public's trust. Wait too long, and you lose the public, literally. The window to get it right can be small when you need the public's help to make for an effective and safe evacuation.

With the millions of people who would need to evacuate during a large eruption from Vesuvius, would such an evacuation even be possible? We tend to think of large-scale and long-term evacuations as things that happen in developing countries, where poor farmers and villagers have to go live in tents: the prototypical refugee camps. An eruption of Vesuvius would require evacuation and temporary shelter on a scale almost unseen in the developed world.

Using the hurricane example again, the largest evacuation in U.S. history was for Hurricane Rita in 2005, when over 3 million people where evacuated from a wide swath of Texas and Louisiana, the majority coming from the Houston metro area. However, this was a temporary evacuation for most people who could head back in a few days after the storm passed. The evacuation of New Orleans for Katrina was "only" in the hundreds of thousands, but might be more similar to a volcanic disaster, where many were not able to return for months to years (if ever).

In both cases, the the Interstate System in U.S. allowed for these mass exoduses from major cities, but still ran into problems of congestion. In Italy, there isn't such a network of large roads to accommodate all that traffic of people fleeing. Even in the face of a potentially lethal eruption, some people will stay behind beyond the warning given by the government. People don't like to leave their homes (just ask Harry Truman).

Remember, it was only 5 years ago when a little volcano in Iceland named Eyjafjallajökull erupted and plunged Europe into chaos because a bunch of people missed their flights. Imagine what could happen if all the Naples had to be evacuated in the face of a restless Vesuvius for months or years?

By some estimates, a large eruption of Vesuvius could kill over 10,000 people, but if a long-term evacuation happens, that number could climb from disease related to housing in temporary shelters. The hit to the Italian economy could be over $20 billion ... and that's likely a low-end estimate that doesn't include the cost of shelter for the potentially millions of volcanic refugees.

So, does this mean it's hopeless? Hardly! Here's what needs to be done to have Naples (or any large city near a volcano) ready for the next big eruption:

Volcano monitoring: This means both having sufficient instruments to measure all the rumbling and burping of the volcano but more importantly, people who are trained to interpret the signals.

Mitigation: Planning needs to be done now for the disaster that may never come in our lifetime. The plan needs to be clear and easy to follow as it is passed on to new people as the years go by. It also needs to be constantly revised as new information becomes available and the city itself changes.

Communication: The scientists need to communicate effectively and clearly to the public about the threat the volcano poses and what different scientific observations mean in terms of eruption. Trust needs to be maintained between the scientists, planners, emergency managers and the public – and communication is the key to that trust.

Practice: Plans are good, but practice and drills are better. A great example of this is Rabaul in Papua New Guinea, where practice saved lives during a full blown eruption of the caldera.

Volcanoes are dangerous. Volcanoes are deadly. But they're also beautiful and awe-inspiring. Being prepared for that next eruption is the key from making the world's most dangerous volcano into the deadliest.