The Lake of Pergusa is the only natural lake in the central Sicily. It is situated among a group of rises belonging to the Erei Mounts. It has been originated by a sinking of the ground, and has always been related to the ancient legend of the rape of Proserpina. This lake has not tributaries neither emissaries; in fact its watering is due only to the atmospheric contribution, therefore it feels the effects of the long periods of dryness. Its blue and shallow waters (they go down until just a little more than 4 metres) are brackish because of the lack of emissaries. Since 1995 the lake of Pergusa and the surrounding area constitute a Special Nature Reserve for the protection (according to the official explanation) of a lake environment giving hospitality to a very rich variety of bird population. It is the only humid oasis in the heart of Sicily for the migratory birds. Between April and May and between October and November Pergusa becomes a regular crossroads of birds, because of its geographical position and because of its being a humid oasis in a landscape dominated for long hours of continuous flight over the sea, from and towards Africa. Periodically the lake, because of a synergy between some of the microscopical organisms living in it, makes register an exclusive phenomenon: its waters turn red. Protagonist of this phenomenon is a little "shrimp" that, in order to protect itself against the rays of the summer sun, becomes tinged with a red pigment and settles by very large colonies under the aquatic plants. The pigment moves to the water until it gives to the lake's mirror a reddish colour.