in the Guadalquivir basin, Seville suffered regular heavy flooding; perhaps worst of all were the floods that occurred in November 1961 when the River Tamarguillo, a tributary of the Guadalquivir, overflowed as a result of a prodigious downpour of rain, and Seville was consequently declared a disaster zone. Trade unionism in Seville began during the 1960s with the underground organisational activities of the Workers' Commissions or Comisiones Obreras (CCOO), in factories such as Hytasa, the Astilleros shipyards, Hispano Aviación, etc. Several of the movement's leaders were imprisoned in November 1973. Recent developments On 3 April 1979 Spain held its first democratic municipal elections after the end of Franco's dictatorship; councillors representing four different political parties were elected in Seville. On 5 November 1982, Pope John Paul II arrived in Seville to officiate at a Mass before more than half a million people at the fairgrounds. He visited the city again on 13 June 1993, for the International Eucharistic Congress. In 1992, coinciding with the fifth centenary of the Discovery of the Americas, the Universal Exposition was held for six months in Seville, on the occasion of which the local communications network and urban infrastructure was greatly improved under a 1987 PGOU plan launched by Mayor Manuel del Valle: the SE-30 ring road around the city was completed and new highways were constructed; the new Seville-Santa Justa railway station had opened in 1991, while the Spanish High-Speed Rail system, the Alta Velocidad Española (AVE), began to operate between Madrid-Seville. The Seville Airport was expanded with a new terminal building designed by the architect Rafael Moneo, and various other improvements were made. The Alamillo Bridge and the Centenario Bridge, both crossing over the Guadalquivir, also were built for the occasion. Some of the installations remaining at the site after the exposition were converted into the Scientific and Technological Park Cartuja 93. In 2004 the Metropol Parasol project, commonly known as Las Setas (The Mushrooms), due to the appearance of the structure, was launched to revitalise the Plaza de la Encarnación, for years used as a car park and seen as a dead spot between more popular tourist destinations in the city. The Metropol Parasol was completed in March 2011, costing just over €102 million in total, more than twice as much as originally planned. Constructed from crossed wooden beams, Las Setas is said to be the largest timber-framed structure in the world.