van Musschenbroek in 1746. Another development was in cryogenics: Heike Kamerlingh Onnes (1913 Nobel prize winner in physics) liquefied helium for the first time (1908) and later managed to reach a temperature of less than one degree above the absolute minimum. Albert Einstein also spent some time at Leiden University during his early to middle career. Leiden today The city's biggest and most popular annual festival is celebrated on 3 October and is called simply 3 Oktober. The people of Leiden celebrate the end of the Spanish siege of 1574. It typically takes place over the course of two to three days and includes parades, a hutspot feast, historical reenactments, a funfair and other events. Since 2006, the city has also hosted the annual Leiden International Film Festival. Leiden has important functions as a shopping and trade centre for communities around the city. The city also houses the Eurotransplant, the international organization responsible for the mediation and allocation of organ donation procedures in Austria, Belgium, Croatia, Germany, Luxembourg, the Netherlands and Slovenia. Leiden also houses the headquarters of Airbus, a global pan-European aerospace and defence corporation and a leading defence and military contractor worldwide. The group includes Airbus, the leading manufacturer of commercial aircraft worldwide. Rivers, canals and parks The two branches of the Oude Rijn, which enter Leiden on the east, unite in the centre of the city. The city is further intersected by numerous small canals with tree-bordered quays. On the west side of the city, the Hortus Botanicus and other gardens extend along the old Singel, or outer canal. The Leidse Hout park, which contains a small deer park, lies on the northwest border with Oegstgeest. The Van der Werf Park is named after the mayor , who defended the city against the Spaniards in 1574. The city was beleaguered for months and many died from famine. The open space for the park was formed by the accidental explosion of a ship loaded with gunpowder in 1807, which destroyed hundreds of houses, including that of the Elsevier family of printers. Buildings of interest Because of the economic decline from the end of the 17th until the middle of the 19th century, much of the 16th- and 17th-century city centre is still intact. It is the second largest 17th-century town centre in the Netherlands, the largest being Amsterdam's city centre. A hundred buildings in the centre are decorated