Complex sentence: Alexander Island was discovered on January 28,1821 by a Russian expedition under Fabian Gottlieb von Bellingshausen, who named it Alexander I Land for the reigning Tsar Alexander I of Russia, but it was believed to be part of the Antarctic mainland until 1940.
Split into simpler sentences: Alexander Island was discovered on January 28,1821 by a Russian expedition under Fabian Gottlieb von Bellingshausen, who named it Alexander I Land for the reigning Tsar Alexander I of Russia. The island was believed to be part of the Antarctic mainland until 1940.