Simplify the following text for a Grade 9 native English-speaking person.
Original: European maps continued to show this hypothetical land until Captain James Cook 's ships , HMS Resolution and Adventure , crossed the Antarctic Circle on 17 January 1773 , in December 1773 and again in January 1774 .
Simplified: European maps continued to show this unknown land until Captain James Cook 's ships , HMS Resolution and Adventure , crossed the Antarctic Circle on 17 January 1773 , in December 1773 . They crossed it again in January 1774 .