In the early 1900s, Riverside Drive almost eclipsed Fifth Avenue as New York City's most opulent millionaire's row. Many free-standing mansions were built along this breezy, leafy road for Gilded Age business barons and titans of industry, with unspoiled views of Riverside Park and the Hudson River.

More than a century later, only two of those free-standing mansions still stand. One is at West 107th Street. Stand might not be the right word; its pristine white marble facade glistens like a jewel.

This is the Schinasi mansion at 351 Riverside Drive. A French Renaissance mini-palace, it was built in 1909 for Morris Schinasi, an immigrant from Turkey who made a fortune importing Turkish cigarettes with his brother, Solomon.

(Solomon also moved into a palatial mansion on Riverside Drive and 89th Street, which was originally built for the Rice family. Coincidentally, Solomon's house is the second surviving free-standing mansion on Riverside.)

The exterior of the Morris Schinasi mansion was and is stunning. Designed by William B. Tuthill, the architect behind Carnegie Hall, the house features fancy dormer windows, a green tiled roof, and bronze balcony grills, according to Landmarks of New York, Fifth Edition.

The Schinasi mansion in 1909, just completed

But there's one feature inside the house that's truly unique, even for a Gilded Age millionaire's mansion: a tunnel from the basement that supposedly leads to the Hudson River.

The tunnel isn't mentioned in newspaper articles or in the Landmark Preservation Commission report designing the mansion as a historic landmark. But apparently, it really does exist.

Morris Schinasi, Turkish tobacco baron

In 2007, the New York Times mentioned the tunnel in a writeup about the mansion, which at the time was put on the market for $20 million by a Columbia University professor. (The professor bought it for an astounding $325K in 1979.)

"Its three floors included an Egyptian marble hall inlaid with Turkish glass, a Louis XVI drawing room, a library, a smoking room and a reception hall," wrote Lily Koppel in the Times piece. "The pineapple, a traditional symbol of hospitality, is found throughout the house, set into the moldings in gold and bronze. Among the house's unusual features is an underground passage to the Hudson River, now sealed."

A second Times story in 2012 by Constance Rosenblum even featured a photo of what might be the tunnel or perhaps a basement passageway leading to it.

Schinasi mansion in 1932

When the tunnel was built, however is puzzling—as is the tunnel's purpose. The Times wondered as well.

"The mansion's most beguiling feature is a tunnel in the basement that was thought to have extended west to the Hudson River," wrote Rosenblum. "But exactly what had the tunnel been used for? To smuggle in Turkish tobacco, or perhaps alcohol or hashish? Or as a conduit for ladies of the evening?"

Every house in New York has its mysteries.

Curious about the Gilded Age mansions that once lined Riverside Drive on the Upper West Side? Join Ephemeral New York on a walking tour Sunday, August 29 that explores the history of Riverside Drive and the short period of time when Riverside rivaled Fifth Avenue as New York City's millionaire colony. Click the link for more details.

[Third image: MCNY, 1909; x2010.28.118; fourth image: Wikipedia; fifth image: NYPL]