Complex sentence: Discovered in the mid- 19th century, they have been territories of Australia since 1947 and contain the only two active volcanoes in Australian territory, the summit of one of which, Mawson Peak, is higher than any mountain on the Australian mainland.
Split into simpler sentences: Discovered in the mid- 19th century, they have been territories of Australia since 1947 and contain the only two active volcanoes in Australian territory. The summit of one, Mawson Peak, is higher than any mountain on the Australian mainland.