The structure known as "Virgil's tomb" is found at the entrance of an ancient Roman tunnel (also known as "grotta vecchia") in Piedigrotta, a district two miles from the centre of Naples, near the Mergellina harbor, on the road heading north along the coast to Pozzuoli. While Virgil was already the object of literary admiration and veneration before his death, in the Middle Ages his name became associated with miraculous powers, and for a couple of centuries his tomb was the destination of pilgrimages and veneration.

Answer this question, if possible (if impossible, reply "unanswerable"): What is the entrance of the ancient Roman tunnel where "Virgil's tomb" is located also called?
grotta vecchia