Sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as the Minyé sinkhole in Papua New Guinea or Cedar Sink at Mammoth Cave National Park in Kentucky, an underground stream or river may be visible across its bottom flowing from one side to the other. Sinkholes are common where the rock below the land surface is limestone or other carbonate rock, salt beds, or in other soluble rocks, such as gypsum, that can be dissolved naturally by circulating ground water. Sinkholes also occur in sandstone and quartzite terrains. As the rock dissolves,
Henry A. Schade obtain and exploit German science and technologies in areas immediately behind the front lines of fighting in Europe. As allied ground forces gained ground, Schade and his scientists would rush into the void left behind advancing forces to study the equipment and weaponry employed by the Germans. The United States Naval Technical Mission in Europe was particularly interested the capture and study of rocketry and all things naval as German ports and bases were wrested from Hitler's forces. Schade initially headquartered the Mission in Paris, but soon the Mission was constantly on the move as allied forces advanced more and