Simplify the following text for a Grade 7 native English-speaking person.
Original: Lake Eyre has been a site for various land speed record attempts on its salt flats , especially those by Donald Campbell with the Bluebird-Proteus CN7 .
Simplified: At Level Post Bay , the crust is more than one metre thick , and because of its flat surface it has been the site for various land speed record attempts . In 1964 , British driver , Sir Donald Campbell , set a new record of 664 k , in the Bluebird-Proteus CN7 .