Passing through the tunnel into the open air can be symbolically compared to

Childbirth can be symbolically compared with a passing through a tunnel into the open air.

Explanation:

The passing through of a tunnel into an open air is like the birth of a child. It goes through a long labor of dark and unsure passage, with unknown and unexpected bumps along the way. But, once the dark passage is cleared, one can finally see the light of the outside world. It will feel like new found freedom, with the world all free and without any boundaries.

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