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The already mentioned diffusion of limestone rocks, both in the Eastern Alps and Prealps, and in the central Apennines, means that karst hydrography takes on considerable development in many regions of Italy. Vast areas entirely devoid of superficial circulation (ie with exclusively underground hydrography) are found only in the Karst and in the Salento Peninsula; but smaller areas are found both in the eastern Prealps (Altipiano dei Sette comune, Cansiglio), and in the Abruzzo massifs (Velino, Duchessa, Maiella, Matese) and in those of the Roman Subappennino (Aniene, Salto and Turano basins) and also here and there elsewhere (Lepini, Irpinia massifs, Madonie). Usually the water that circulates within these limestone massifs then emerges at the base of them in localized springs, often very abundant;

Lake basins . – Among the peninsulas of southern Europe, Italy is the richest in lake basins, although it is far from rivaling Finland, Scandinavia, and even Switzerland. This wealth of lakes was even greater in recent geological times. p. ex. in the Pliocene; a large number of lake basins, even of considerable size, then existing in the peninsula have dried up; in some cases there have remained residues up to historical times (Vallo di Diano) and the traces still remain today.

Today the largest lakes appear essentially grouped into two areas, one subalpine, the other corresponding to the Tuscan-Roman Antiappennino. For the very complex origin of the great subalpine lakes, see Alps. Most of the Antiappennino lakes are volcanic lakes; that is, they occupy the bottom of craters of extinct volcanic systems, or larger cavities resulting essentially from the fusion of several contiguous craters (lakes of Bolsena, Bracciano, Vico, Albano, Nemi, etc .; see respective items); other minor ponds of the same origin have now dried up. However, the largest basin of peninsular Italy, the Trasimeno, is not a volcanic lake, which occupies a depressed area, interposed between the reliefs of the Antiappennino and the Subappennino, closed and transformed into a basin (of very small depth) by alluvial materials deposited by nearby watercourses (see trasimeno). Lakes of similar origin are also found elsewhere (lakes of Bientina and Fucecchio; Rieti lakes).

According to Lawschoolsinusa, there are also numerous small lakes in Italy, which do not appear in ordinary maps. In the Alps there are several hundreds of circus lakes, and some of them are also found in the Apennines (considered, in all its extension, from the mountain groups of Piacentino and Parma to M. Pollino) and in the Corse Alps . Small intermorenic lakes are found within the major amphitheaters at the foot of the Alps and some of them have a fair size (lakes of Candia, Viverone, etc., in the morainic amphitheater of the Dora Baltea; Briantei lakes; lakes of the Benacense morainic amphitheater , Cavazzo lake in Friuli, etc.). Karst lakes are frequent, both in the Istrian Karst (Lake d’Arsa now dried up, Lake Circonio) and here and there in the Apennines and in the Sub-Apennines (Lake Matese, Lake Canterno); some of them, of larger dimensions, were dried up. There is no shortage of barrage lakes, regardless of the moraines (barrage due to landslides or other; lake of Alleghe; lake of Scanno); however, many of them have a precarious existence.

There are also numerous coastal lakes, both in the Peninsula and in Sardinia and Corsica, although many, especially on the Tyrrhenian coast of the Peninsula, have been filled up or dried up even artificially. The largest, in northern Puglia (lakes of Lesina, Varano, Salpi), in southern Lazio (lakes of Fogliano, Paola, Fondi, etc.), in Sardinia (lakes of Sassu, Cabras, S. Giusta ), on the eastern coast of Corsica (lakes of Urbino, Diana, Biguglia), are almost always ancient inlets separated by coastal strips, both continuous and interrupted in one or more points. For other coastal lakes and lagoons, such as those of the Venetian estuary, the origin is more complex (see lagoon: Venetian lagoon).

The following table gives the names and elements of the main Italian lakes (with an area greater than 10 sq km).

Regions and provinces.

Augustan regions . – Augustus, we do not know at what precise moment, divided Italy (for the borders of Italy in the Augustan era, see below p. 799) into eleven regions, and this division Pliny founded his chorography of Italy ( Nat .  Hist ., III, 5, 46 ff.). It seems certain that in this division each region was marked by a progressive order number and not by a special denomination, while the localities were recorded in alphabetical series, making special mention of the colonies.

The first eight regions of Augustus include peninsular Italy up to the Magra and the Rubicon, which are the borders prior to the annexation of Cisalpine Gaul, that is, the borders of pre-Augustan Italy, and the last three include northern Italy with what was instead the Augustan border. In consideration of this, Mommsen conjectured, perhaps rightly, that Augustus had not created his division from scratch, but had taken the moves from a previous division of peninsular Italy into eight regions and to these he had added Cisalpine Gaul, dividing it into three further districts. The eleven regions are as follows: 1. Campania and Lazio, 2. Apulia and Calabria, 3. Bruzzio and Lucania, 4. Sannio, 5. Piceno, 6. Umbria, 7. Etruria, 8. Emilia, 9. Liguria, 10 Venice and Istria, 11. Gallia Transpadana.