Simplify the following text for a Grade 6 native English-speaking person.
Original: The first European to penetrate the mythical barrier was A. C. Gregory from the north in March 1858 ; later the same year , an expedition under B. H. Babbage and Major Warburton in the north-west also crossed the non-existent barrier near modern Marree .
Simplified: The first European to get through this barrier was A. C. Gregory from the north in March 1858 . Later in 1858 , an expedition under B. H. Babbage and Major Warburton in the north-west also crossed the non-existent barrier near modern Marree .