求翻译：After the sun rose, the balloon went higher. It went up to 3,000 metres, and the air was very cold. The water in the balloon became ice. Snow fell past the men's basket, and they could see more snow on the ground. The men tried to throw out some more sand; but it was hard. They tried to break the icy sand with their knives, but it was not easy. The work was slow and they were still falling, so they had to drop some whole bags of sand. One of them fell on an icy lake and made a black hole in the ice是什么意思？

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