It's a cave.
A passage through a hill ; underground, is called a TUNNEL. However, a passage between two hills (surface) is a VALLEY. A passage between two hills is dependent on its width. If the hills are low and far apart, then it is a VALE. However, if the hills are high and close together then it is a GLEN or RAVINE, or GORGE.
Sideling Hill Tunnel was created in 1940.
The address of the Tunnel Hill Historical Foundation is: Po Box 114, Tunnel Hill, GA 30755-0114
A passage through a hill is typically called a tunnel. It is an underground passageway that cuts through the hill, allowing for easier transportation or access.
A tunnel
Cobble Hill Tunnel was created in 1844.
Squirrel Hill Tunnel was created in 1953.
Hunsbury Hill Tunnel was created in 1881.
Rays Hill Tunnel was created in 1940.
Lowry Hill Tunnel was created in 1971.
Southwick Hill Tunnel was created in 1996.
Laurel Hill Tunnel was created in 1940.