• cave
  • 1) An underground hollow with access from the ground surface or from the sea, often found in limestone areas and on rocky coastlines. 2) A natural cavity, chamber or recess which leads beneath the surface of the earth, generally in a horizontal or obliquely inclined direction. It may be in the form of a passage or a gallery, its shape depending in part on the joint pattern or structure of the rock and partly on the type of process involved in its excavation. Thus, caves worn by subterranean rivers may be different in character from, and of considerably greater extent than, a sea-cave eroded by marine waves. 3) A natural underground open space, generally with a connection to the surface and large enough for a person to enter. The most common type of cave is formed in a limestone by dissolution.
Abstract from DBPedia
    A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word cave can also refer to much smaller openings such as sea caves, rock shelters, and grottos, though strictly speaking a cave is exogene, meaning it is deeper than its opening is wide, and a rock shelter is endogene. Speleology is the science of exploration and study of all aspects of caves and the cave environment. Visiting or exploring caves for recreation may be called caving, potholing, or spelunking.

    洞窟(どうくつ、英: cave, grotto, cavern)とは、地中にある一定の大きさの空間。洞穴(どうけつ、ほらあな)とも言う。一般には地下空間のうち人間が入ることが可能なものをいい、洞口の長径が奥行きよりも小さければ洞長2m程度でも洞窟と呼ばれる。 水平方向に伸びている横穴や井戸状に開口している縦穴(竪穴)などがある。通常、洞内空間は大気で満たされている。内部の気温は、一般に洞窟がある外部の平均気温になり、内部は外部と較べると夏は涼しく・冬は暖かい。また地中であることも含め、一般に湿度が高い。洞口部では日光が差し込むこともあるが、氷河洞・雪渓洞などを除いて奥部は完全な暗黒となる。洞内空間が地下水・海水・堆積物で満たされている洞窟もある。完全に水没している洞窟は水中洞窟、特に海面下に沈んだものは沈水洞窟と呼ばれる。 広義には自然洞窟だけでなく人工洞窟や混成洞窟を含む。