Can you give some perspective? How high are the peaks off the ocean floor, 1,000 feet or so? Less?
A monster! 3000 meters high, or ~10000 feet and 500 under sea.
“Mt. Marsili, which rises 9,800 feet (3,000 meters) from the seabed in the Tyrrhenian Sea southwest of Naples, is capable of producing huge tidal waves along southern Italy if its activity ever does build to an eruption. According to the researchers, “Il Marsili’” is about 40 miles (65 km) long and 25 miles (40 km) wide. The experts said its tip was only 1,640 feet (500 metres) below the surface of the water. “
“The most recent data concerning the Tyrrhenian confirm certain only apparently surprising facts. To begin with, the tallest volcano in Europe is not Etna, but Marsili, which is also the largest, being 65 kilometres long and more than 3000 meters high. “ http://www.geologia.com/english/fi2004/2004_tirreno.html
“The undersea Marsili, 9,800 feet tall and located some 90 miles southwest of Naples, has not erupted since the start of recorded history. It is 44 miles long and 19 miles wide, and its crater is about a quarter mile below the surface of the Tyrrhenian Sea.”
http://www.foxnews.com/scitech/2010/03/29/undersea-volcano-threatens-italy/?test=latestnews