La Tana di Cascaltendine
P. Magri, 1880

"The wide entrance to the cave opens up at the base of an enormous hollow in the mountain which has the general shape of a parabolic curve, though this is somewhat irregular and imperfect lower down on the left-hand side."

Note - this picture is taken standing at the entrance to the cave facing outwards and looking straight up.
 
Parabolic curve
 
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