A tunnel is a passage that runs underground or through something, like a train tunnel that cuts through a mountain. Some theme parks have networks of underground tunnels so that employees can move around out of sight of visitors.
Some tunnels, like New York's Lincoln Tunnel and the Holland Tunnel, which connect New York City to New Jersey, are large and solid enough to drive cars through. Others are much smaller, like the tunnels small animals dig through snow or soil for safety and shelter. As a verb, tunnel means "dig a tunnel" or "force through," the way your neighborhood groundhog tunnels under your dad's vegetable garden.