We could have said the mortar was a different color where they filled the tunnel opening.

Any movement near the tunnel openings were met with enemy fire.

He poked his head cautiously out of the tunnel opening.

A moment later they saw two figures wriggle through the tunnel opening.

You sprint off to the tunnel opening and watch for the train to go back in again.

She said nothing further, merely followed him to the tunnel opening.

And then there was another scent, as she passed a tunnel opening.

They had not come too far from the tunnel opening.

The tunnel opening was halfway up one wall of the chamber.

Free, she swam to the tunnel opening and ducked through.