The jaws of a tunnel, etc.
Definition 1 of 2

Visual Imagery
Imagine the jaws of an animal; the mouth of the tunnel is similar and suggests entry.

The train disappeared into the jaws of the tunnel.
Metaphorical Use
Sometimes 'jaws' can imply danger or challenge about to be faced.

She hesitated at the jaws of the tunnel, sensing danger.
Non-literal Context
Used figuratively, it doesn't always describe a physical tunnel but rather an entrance to an experience.

At the jaws of adulthood, he felt both excited and scared.
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