Todor Zhivkov to 1988, Bulgaria won an unprecedented 153 Olympic medals and numerous European and world competitions in sports as diverse as volleyball, rhythmic gymnastics and wrestling. Dissent was punished under Zhivkov's rule. The CSS was a feared tool of control, and overt opposition largely stayed underground until the late 1980s. In 1978, the Bulgarian dissident Georgi Markov was assassinated in London by an agent who stabbed him with an umbrella tip which implanted a very small ricin ball. According to former KGB general Oleg Kalugin, this was requested by Zhivkov and either performed by the KGB or it assisted the CSS;
Umbrella gas used by riot police. In the 1950s Frei Otto transformed the universally used individual umbrella into an item of lightweight architecture. He developed a new umbrella form, based on the minimum surface principle. The tension loaded membrane of the funnel-shaped umbrella is now stretched under the compression-loaded bars. This construction type made it technically and structurally possible to build very large convertible umbrellas. The first umbrellas of this kind (Federal Garden Exhibition, Kassel, 1955) were fixed, Frei Otto constructed the first convertible large umbrellas for the Federal Garden Exhibition in Cologne 1971. In 1978 he built a group of