Suddenly they came to a waterfall which dropped into an underground lake at the bottom of the cave.

突然，他们来到一道瀑布前，那瀑布奔泻而下，注入洞底一处地下湖里。

And another news an underground lake beneath the south pole on Mars may indicate a network of salty ponds.

另一则新闻是，火星南极下的地下湖可能表明有一个盐湖网络。

They plunged even deeper, passing an underground lake where huge stalactites and stalagmites grew from the ceiling and floor.

他们已经冲到地底下更深的地方，经过一片地下湖，巨大的石钟乳从洞顶垂下，巨大的石笋则在地面生长。

It used to be an underground lake or an underground river, but that water moved on to some other place, leaving behind or having already cut this cave in the rock, right?

本来是个地下湖或者地下河，但是那个水流到别的地方去了， 留下或者已经把这个岩洞给挖出来了吧？

Of course, unlike the plentiful oxygen we breath, only 2.5% of all Earth's water is fresh; most of which is locked up in glaciers, ice caps and underground lakes known as aquifers, leaving less than 1% available for drinking.

当然，与我们呼吸的充足氧气不同，地球上只有 2.5% 的淡水；其中大部分被锁在冰川、冰盖和称为含水层的地下湖泊中，只有不到 1% 可供饮用。