The government believed such expenditures could be covered by foreign borrowing and increased sales of its mineral wealth in the international market. North Korea invested heavily in its mining industries and purchased a large quantity of mineral extraction infrastructure from abroad. It also purchased entire petrochemical, textile, concrete, steel, pulp and paper manufacturing plants from the developed capitalist world. This included a Japanese-Danish venture that provided North Korea with the largest cement factory in the world. However, following the world 1973 oil crisis, international prices for many of North Korea's native minerals fell, leaving the country with large debts and an inability to pay them off and still provide a high level of social welfare to its people. North Korea began to default in 1974 and halted almost all repayments in 1985. As a result, it was unable to pay for foreign technology. Worsening this already poor situation, the centrally planned economy, which emphasized heavy industry, had reached the limits of its productive potential in North Korea. "Juche"s repeated demands that North Koreans learn to build and innovate domestically had run its course as had the ability of North Koreans to keep technological pace with other industrialized nations. By the mid to late-1970s some parts of the capitalist world, including South Korea, were creating new industries based around computers, electronics, and other advanced technology in contrast to North Korea's Stalinist economy of mining and steel production. Migration to urban areas stalled. Despite the emerging economic problems, the regime invested heavily in prestigious projects, such as the "Juche" Tower, the Nampo Dam, and the Ryugyong Hotel. In 1989, as a response to the 1988 Seoul Olympics, it held the 13th World Festival of Youth and Students in Pyongyang. In fact, the grandiosity associated with the regime and its personality cult, as expressed in monuments, museums, and events, has been identified as a factor in the economic decline. In 1984, Kim visited Moscow during a grand tour of the USSR where he met Soviet leader Konstantin Chernenko. Kim also made public visits to East Germany, Czechoslovakia, Poland, Hungary, Romania, Bulgaria and Yugoslavia. Soviet involvement in the North Korean economy increased, until 1988 when bilateral trade peaked at US$2.8 billion. In 1986, Kim met the incoming Soviet leader Mikhail Gorbachev in Moscow and received a pledge of support. However, Gorbachev's reforms and diplomatic initiatives, the Chinese economic reforms starting in 1979, and the collapse of the Eastern Bloc from 1989 to 1991 increased North Korea's isolation.