Lava tubes are partially empty, cave-like conduits underground, formed during volcanic eruptions by flowing and cooling lava.
Natural Tunnel State Park (Virginia, US) features an 850-foot (259 m) natural tunnel, really a limestone cave, that has been used as a railroad tunnel since 1890.
Punarjani Guha in Kerala, India. Hindus believe that crawling through the tunnel (which they believe was created by a Hindu god) from one end to the other will wash away all of one’s sins and thus allow one to attain rebirth. Only men are permitted to crawl through the tunnel.
Torghatten, a Norwegian island with a hat-shaped silhouette, has a natural tunnel in the middle of the hat, letting light come through. The 160-metre (520 ft) long, 35-metre (115 ft) high, and 20-metre (66 ft) wide tunnel is said to be the hole made by an arrow of the angry troll Hestmannen, the hill being the hat of the troll-king of Sømna trying to save the beautiful Lekamøya. The tunnel is thought actually to be the work of ice. The sun shines through the tunnel during two few minutes long periods every year.
Small "snow tunnels" are created by voles, chipmunks and other rodents for protection and access to food sources. For more information regarding tunnels built by animals, see Burrow.