Complex sentence: In 1934/35 Danish-born Caroline Mikkelsen, wife of Captain Klarius Mikkelsen, sailed to Antarctica and landed on 20 February 1935 and was, until recently, thought to be the first woman to land on Antarctica.
Split into simpler sentences: In 1934/35 Danish-born Caroline Mikkelsen, wife of Captain Klarius Mikkelsen, sailed to Antarctica and landed on 20 February 1935. At the time, and until recently, Mikkelsen was thought to be the first woman to land on Antarctica.