Some Russian soldiers crawled through the pipeline and waited several days for orders to attack, a Russian military blogger ...

They're called the Cu Chi Tunnels, and tourists travel from around the world to crawl inside. They love ducking below camouflage trap doors and cramming themselves in the tight passageways.

Put toys at one end of the tunnel and the baby at the other end. Show the toys to the baby and encourage him to reach out to them. The baby will crawl through the tunnel to the other end.