The Common Frog is found throughout much of Europe as far north as well north of the Arctic Circle in Scandinavia and as far east as the Urals, except for most of Iberia, southern Italy, and the southern Balkans.
Common Frogs are active almost all of the year, only hibernating when it gets very cold and the water and earth are consistently frozen. Common frogs hibernate in running waters, muddy burrows and can hibernate in layers of decaying leaves and mud at the bottom of ponds. The fact that they can breathe through their skins allows them to stay underwater for much longer periods of time when they are hibernating. Common Frogs breed in shallow, still, fresh water such as ponds, with breeding commencing in March. The adults congregate in the ponds, where the males compete for females. The courtship ritual involves croaking, and a successful male grasps the female under the forelegs. During the mating season the males can be recognised by a darkened swelling, the nuptial pad on their 'thumbs'. The females, which are generally larger than the males, lay between 1,000 & 2,000 eggs which float in large clusters.
Source: en.wikipedia.org/wiki/Common_Frog - 21.05.2010
It is fascinating, that such an animal, after living on land for months, is able to spend the winter sleeping underwater in the bottom of a little water.