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At times, like an underground river, it is even lost from sight.

The cool, moist air of an underground river came up to meet him.

"Oh, this is the other end of the underground river!"

"How will we get through the lake and the underground river?"

The name later given to the underground river beneath the desert.

The project to use the water from the underground river for irrigation.

There must be an underground river running through the mountain.

Does the well lead to an underground river, is that where you came from?

Another underground river, and this one not of his world.

Very well, but the underground river is the source of that particular myth.