Then he lay down and blasted a horizontal tunnel several feet below the surface.

The four companionways to the deck above were horizontal tunnels.

The mountain is criss-crossed by horizontal tunnels on six levels.

He was facing a long, horizontal tunnel cut through the earth.

The 7th Squadron men had arrived at the other end of the horizontal tunnel.

He had reached the beginning of a horizontal tunnel.

There were no sounds in the horizontal tunnels that led off from the central stairway at either of the two main levels.

They found the White Room off to the left of a horizontal tunnel at the second level below the surface.

I added, "Could they be driving a new horizontal tunnel just under the surface?"

He crawled down the horizontal tunnel again and saw the plug was still in place.