In essence the east side of Italy features a fold and thrust belt raised by compressional forces acting under the Adriatic Sea. This side has been called the "Apennine-Adriatic Compressional Zone" or the "Apennines Convergence Zone." On the west side of Italy fault-block mountains prevail, created by a spreading or extension of the crust under the Tyrrhenian Sea. This side is called the "Tyrrhenian Extensional Zone." The mountains of Italy are of paradoxical provenience, having to derive from both compression and extension: