Zonnestraal is one of Duiker's architectural marvels, and among the best examples of the New Objectivity style. The estate was submitted to UNESCO's list of World Heritage Sites, and is currently on the tentative list. Zonnestraal was built as a tuberculosis sanatorium in the 1920s and 1930s. It consists of one large main building, two smaller pavilions and some additional buildings that were added later. The buildings are made of concrete and glass, in the colours white, black and a specific kind of light-blue called 'Duiker blue'. Start at Brasserie Zonnestraal (Loosdrechtse Bos 15, +31 35 538-5402), buy the walking guide there for €5 and follow the 4,5 km walking trail around the estate. If you want to see the interior of the building, you can only do so by taking the guided tour that is given every last Sunday of the month. You need to register for the tour beforehand by calling the listed phone number or by sending an e-mail. The tour starts at Brasserie Zonnestraal at 12:00 and 14:00, and costs €8, to be paid in cash to the tour guide. The tour takes 90 minutes.