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
Hunsbury Hill Tunnel was created in 1881.
Rays Hill Tunnel was created in 1940.
Cobble Hill Tunnel was created in 1844.
Laurel Hill Tunnel was created in 1940.
Southwick Hill Tunnel was created in 1996.
Lowry Hill Tunnel was created in 1971.
Stowe Hill tunnel was created in 1838.