Sphinx Tunnel is a term referring to several secret passages within the Leyendecker Museum. Presumably the product of the architectural eccentricity of the museum's architect, Arvin Slatherlord Loudermilk III (who was fond of mixing different architectural styles together), they are clearly inspired by Egyptian architecture.
There are four known Sphinx Tunnels in the museum.
One leads from Olympia's Office to the Alcoholic Preservation Lab.
Another leads from Wolf Heimlich's Office to Dr. Carrington's Office.
Another can be found in the Mammology Lab leading to the Egyptian Exhibit. Unlike the others, it cannot be opened in the same room as the entrance. Instead, one must find the means of opening it within Ernie Leach's Office, and then proceed to the Mammology Lab immediately after hearing the sound of the passage opening.
The fourth tunnel can be found in the Furnace Room. It leads from the Furnace Room up to the Tyrannosaur Exhibit. An alternate exit leads straight to the furnace, and is not recommended.