The Val del Bove has the shape of a great pit five miles in diameter. It has almost vertical walls, the height of which varies with their position. Those which reach highest up the mountain vary from 3,000 to 4,000 feet in height. Like Vesuvius, Etna has been split or fissured into great crevices that have been filled with lava during the many eruptions of its central crater. On hardening, these lava streams form what are known as dikes. As the sides of the mountain are worn away by erosion, the dikes, being harder than the rest of the cone, project from its sides like huge walls. An excellent opportunity for seeing them is afforded in the walls of the Val del Bove. Sir Charles Lyell, the English geologist, who has carefully studied Mt. Etna, asserts that this mountain began to be formed during a geological period known as the Tertiary Age, through a crater that opened on the floor of the Mediterranean Sea. The material thus thrown out, collected around the crater and produced a mountainous pile that gradually emerged above the level of the sea, and on fresh materials continuing to be thrown out, at length reached its present height. It would appear that at some former time in its history, there were two vents near the top of the mountain, the second crater being formed immediately under the Val del Bove. Soon, however, the second and lower crater was closed, the upper one alone remaining active. The mountain, therefore, continued to be slowly raised in the air by the materials brought out through this opening. Then came the great explosive eruption during which the side of the mountain was blown off to form the great chasm of the Val del Bove. Because of its almost constant activity, Mt. Etna must have been well known to the ancients, who described some of its most violent eruptions. The following brief notes concerning these eruptions have been taken from Lyell. According to Diodorus Siculus, an eruption that occurred before the Trojan war, caused the people living in districts near the mountain to seek new homes. Thucididies, the Greek historian, states that in the sixth year of the Peloponnesian war, which would be about the spring of 425 B. C., a lava stream caused great destruction in the neighborhood of Campania, this being the third eruption that had occurred in Sicily since it had been settled by the Greeks. Seneca, during the first century of the Christian Era, calls the attention of Lucullus to the fact that during his time Mt. Etna had lost so much of its height that it could no longer be seen by boatmen from points at which it had before been readily visible.