Then, unexpectedly, the tunnel dipped and, with a cry, he found himself tumbling head over heels, coming to a jolting halt against a wall.

She'd expected a steady upward climb, but suddenly the tunnel dipped and steps went down beside a white race of water.

At the same time, the tunnel dipped slightly deeper.

Near the end, the tunnel dipped underground.

The tunnel dipped under the electrified barrier and rose on the other side.

The tunnel dipped and turned in a bewildering maze through the bowels of the earth.

Every so often the tunnel dipped underground, and once it ran along the ceiling of the platform booth tunnel system for an extended period.

And now the tunnel dipped very steeply indeed, and went downwards for a long way.

Fifty yards beyond that, the tunnel dipped slightly, widened considerably, and then continued al-most straight up.

The tunnel dipped left then right, took a last turn, and there was an arc of light straight ahead: the end of the tunnel.