the mouth of the

The mouth of something can mean the place where a river flows into the sea or a larger body of water. For example:

We‘ve got to get our hero,

Captain Willard,

to the mouth of the Nung River

 so he can go pursue Colonel Kurtz.

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It was here

that Homer described the Greek encampment 

at the mouth of the Scamander River.

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Located at the mouth of the Bay of Bengal, 

this coastal village was built on top of the Meghna River delta.

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It can also mean the opening of a cave or other underground cavity. For example,

To give you an idea of scale, 

this is looking down towards the mouth of the Beardmore Glacier.

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