A hole in the side of a hill or under the ground is known as Cave.
A large hole in the side of a hill or mountain or in the ground is called a cavern or a cave. These natural formations can be created through various processes like erosion, weathering, or volcanic activity.
A "canyon" is a large hole on the side of a hill or mountain or on the ground. Canyons are typically formed by the erosion of water or wind over time, creating deep valleys with steep sides.
A crater - I don't think there are any other answers possible!
Foxes live in an under ground burrow called a den. It is much like a rabbits hole with two openings for escaping. But a fox's den can be in the side of a hill, mountain, or cliff.
cave
The word you are looking for is "cave." A cave is a hollow opening or natural underground space that is typically large and found in hills, mountains, or the ground.
A Cave
Holes underground are called caves. They could be in the side of a hill. The places where bears live are called dens. A bear's den may be in a cave but often is not.
A large hole in the ground or in the side of a hill or mountain is commonly referred to as a "canyon." Canyons are typically formed by the erosion of water over time, creating deep, narrow valleys with steep sides. They can vary in size from small and narrow to large and expansive, such as the Grand Canyon in the United States.
there is a little hole right under the hood next to the driver side door
give it a name for the location, woody [in the woods] down&out [vally] hiview I call mine the hole, in a hill side,
in the tank. There is an access hole under the rear seat on drivers side