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Since the dawn of human history, caves have played an important role in the life of humankind. Ever since prehistoric humans first ventured into these natural shelters in the rocks, they have been fascinated by the bewildering variety of forms and natural beauty encountered in these caverns. At least 16 different countries have recognized humankind's interest in caves by depicting various aspects of caves.

Caves can be classified by mode of origin. The most common type of cave is formed by the dissolving action of water seeping through joints or fractures in a region underlain by limestone, dolomite or marble. Such a region often has a characteristic from of irregular depressions, sinks, hummocks, etc., and is called "karst" topography. A typical karst plateau is featured on Japan #664. Carlsbad Caverns, Luray Caverns, etc. and the cave areas of Kentucky and Indiana are especially notable in this country for the abundance of caves. Other examples of limestone caves are Cuba #328, Vatican #55 to 57, Japan $665. Lebanon #291 to 295 and C322 to C325.

Caves may be formed in volcanic regions where molten lava has flowed down over the surrounding country. Rapid cooling of the surface of the lava thickens and hardens into solid rock while still molten lava flows beneath the surface. As lava usually flows in channels, much as a river does, obstructions may sufficiently delay a stream in its course so that part of the flow is diverted and a cavity is left behind in the original course. Gigantic air bubbles may be entrapped in the lava and be broken only after the surrounding lava has cooled. Niue # 101 is an example of a volcanic cave.

Caves may also be formed by the action of the sea pounding on cliffs and forcing water through fractures in the rocks, eroding caves. Sometimes lava pouring into the sea will also form a cavity where steam formed by the lava and the water is entrapped and unable to escape before the surrounding lave cools. Such caves may be practically dry at low tide but full spectacularly at high tide. An example of a sea cave is shown on Tonga #103.

The wind, under desert conditions, may blow against a cliff with such a force that sand and salt picked up in the course of its travels may scour the face sufficiently to produce a cave. Such caves, as these are a favorite stopping place in the desert for sun weary travelers. They may be enlarged and turned into dwellings, as has been the case at Mesa Verde (U.S.A. #743). This cave may have been formed partially by the wind and partially by water at a time when the stream that carved the canyon was at a higher level.

Caves may be filled during the long passage of geological time or may be eroded from two sides in such a manner that a natural bridge or arch is formed. Chile #C212 and Lebanon #CB5and CB6 are examples of destruction of caves.

Caves have been an important source of much of the archeological information that has helped us to understand our primitive ancestors. Beautiful and fascinating drawings have been found on the surfaces of caves in all parts of the world, notably France, Spain and Africa. In historic times, the birthplace of Jesus was probably in a stable in a cave outside the inn, common at that time and place. His tomb was probably in an enlarged natural cave. The early Christians in Rome hid and held meetings in the Catacombs, a natural labyrinth of caverns, which have been artificially enlarged by man (Vatican 55 to 60). Today caves are an overwhelming tourist attraction the world over and are visited by thousands daily. Yet many caves are still unknown, awaiting a fortuitous accident or further erosive processes to uncover them and reveal their full beauty to the world.