Today the natural tunnel was full of water from floor to roof.

A railroad was constructed through the natural tunnel in 1893.

The boats entered a natural tunnel, wide but with a low ceiling.

Although the castle was empty now, I could enter it at will through the natural tunnel.

From the outside, the backway was a natural tunnel in the rock above the falls.

A narrow neck of rock, with a natural tunnel through it, joins the two parts.

It is known for its characteristic hole, or natural tunnel, through its center.

The mail went to Mexico through a long tunnel, partly artificial but mostly natural, which led right under the international border.

"It's a natural tunnel," she volunteered in a different tone.

They stepped into a huge natural tunnel that had been improved by human hands.