This massive, winding river in Namibia flows mostly underground, emerging only for a few days per year. Despite its ephemeral journey, it has left a lasting impression on the Earth's surface.

According to USGS:

Namibia's Ugab River only flows above ground for a few days each year. The subterranean waters underlying this ephemeral river, however, are shallow enough in places to fill hollows and sustain a wildlife population that includes the rare desert elephant.

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Learn more on the USGS website