See inside Kykuit, a 40-room mansion in New York that once belonged to the richest man in the world

Kykuit Mansion.

Talia Lakritz/Business Insider

• Kykuit, a 40-room mansion in Sleepy Hollow, New York, belonged to the Rockefeller family.
• It was built in 1913 for John D. Rockefeller, founder of the Standard Oil Company.
• The property features an art gallery, sprawling gardens, and a collection of vintage cars.

At the height of his success, John D. Rockefeller was the world's first billionaire.

With a net worth of $1.4 billion in 1937, equivalent to around $31.9 billion today...