Hokkaido means literally Northern Sea Circuit or Northern Sea Region The prefecture is the northernmost and largest, and contains Hokkaido island, the second-largest island of Japan, and some tiny islands around. The northern location makes this Japan's coldest region, cool summers and icy and snowy winters. The northern portion of the island belongs to the taiga biome with significant snowfall. Unlike the southern islands it has no wet season (June-July). As a result it is quite popular for fleeing the rain weather, skiing in winter, and for fleeing the summer heat in summer. To make this easier it is connected to Honshu by the undersea railway Seikan Tunnel.
The center of the island is mountainous, with volcanic plateaux. The mountains are surrounded by multiple huge coastal plains such as the Ishikari Plain, Tokachi Plain, the Kushiro Plain, and Sarobetsu Plain. The Kushiro Plain is the largest wetland in Japan. All in all not a good situation for caves, there are some small patches with limestone and caves, and numerous sea caves. There are few subterranea and mines, all in all the number of underground sites is very low for the size of the prefecture.