 Complete the following phrase:  Kolumbo is an active submarine volcano in the Aegean Sea in Greece, about 8 km northeast of Cape Kolumbo, Santorini island. The largest of a line of about twenty submarine volcanic cones extending to the northeast from Santorini, it is about 3 km in diameter with a crater 1.5 km across. It was discovered when it breached the sea surface in 1649-50, but its explosion was not to be compared to the well-known 