Simplify the following text for a Grade 6 native English-speaking person.
Original: Although not the most southerly point of South America , ( which are the Diego RamÃ rez Islands ) Cape Horn marks the northern boundary of the Drake Passage ; for many years it was a major milestone on the clipper route , by which sailing ships carried trade around the world .
Simplified: The Drake Passage is directly to the south of it . For many years , it was a major milestone on the clipper route by which sailing ships carried trade around the world .