Puerto Rico's mountainous backbone is the easternmost extension of a tightly folded and faulted ridge that extends from the Central American mainland across the northern Caribbean to the Lesser Antilles. While the highest point on the island reaches only about 4,389 feet (1,338 metres) at Mount Punta, there is a marine trough north of San Juan that plunges to more than 30,000 feet (9,144 metres) below sea level, one of the lowest ocean depths. The great difference in crustal elevations illustrates the strong tectonic forces that have operated in geologic history to create these features. Puerto Rico still occasionally suffers from earthquakes, reflecting the ongoing geologic processes. Rectangularly shaped, the island measures, at most, only about 111 miles from east to west and a mere 40 miles from north to south. Two valuable islands off the east coast, Vieques and Culebra, are also parts of Puerto Rico, as is the island of Mona to the west.