The subterranean fermentation of which Tuscany is the scene is no doubt due in a large measure to the changes which have taken place in the relative proportions of land and sea. Several isolated hills rise near the coast like islands from the sea, and these have evidently been joined to the mainland by the alluvial deposits brought down by the rivers. The Monti Serra (3,000 feet), to the east of Pisa, between the Arno and the Serchio, are almost insulated even now, for they are surrounded by swamps, and the level of the Lake of Bientina, at their eastern foot, is scarcely thirty feet above that of the Mediterranean. The heights along the coast to the south of Leghorn are not quite so isolated, but the lowland which connects them with the table-land of the interior is only of small elevation. The promontory, however, whose extremities are occupied by the towns of Populonia and Piombino (653 feet), is joined to the mainland only by a low plain of sand. The most perfect type of these ancient islands is presented to us in the superb Monte Argentaro, at the southern extremity of the Tuscan littoral, which rises boldly from the sea to a height of 2,085 feet, and is attached to the mainland by two narrow strips of land covered with pine-trees, enclosing a lake of regular shape: in the midst of it, on a fragment of the ancient beach, is built the town of Orbetello. This lake, which looks almost as if it were the work of a generation of giants, has been converted into an eel-pond, and millions of fish are caught in it every year. Towards the west of this mountain, in the direction of Corsica, lie the islands of Giglio and Monte Cristo (2,062 feet) and the rock of Formica. The island of Elba, farther north, forms a small world of its own.