For four decades, Dutch businessman Ant Palmen bought classic cars that impressed him, eventually building up a collection of 230 vehicles that he hid in two warehouses and an abandoned church in the Dutch city of Dordrecht.

But now for the first time his mysterious collection is being revealed, as the vehicles will be sold next month in an online auction organized by Classic Car Auctions in collaboration with the Aaldering Gallery.

Ant Palmen’s interest in classic vehicles was born in the mid-1960s while working at a car dealership. The first rare vehicle he bought was a yellow Lancia B20. Over the years, it has acquired a wide range of makes and models, from well-known automakers such as Jaguar and Ford to lesser-known ones such as Moretti and Imperia.

He kept the cars as they were when he bought them, moved them to his warehouses and didn’t sell any. He rarely showed the collection to anyone, so very few knew of its existence. Maintenance was mainly done by him. You can definitely call it one of the best kept secret car collections in Europe, says Classic Car Auctions.

Ant Palmen who founded a car parts business had a particular preference for Italian four-wheelers, Alfa Romeo, Lancia, Maserati and Ferrari. His collection also includes French Facel Vega, German BMW, Mercedes and NSU. British cars are represented by Jaguar, Aston Martin and Rolls-Royce, while American classics in the collection include Chevrolet, Cadillac and Ford.

Due to his age and various circumstances, Ant Palmen has decided to sell his collection at the auction that will be held from May 19th to June 7th. Among her rarest—and perhaps most expensive—vehicles is a 1955 Lancia B24 Spider America. Because so few were built, this particular car is expected to sell for between $660,000 and $770,000, according to CNN.