Perhaps a languages teacher could answer this better. However, "to travel around" suggests no fixed purpose or destination e.g. to just go from one country to another - France, Germany, Austria, Italy, etc.... "To travel across" is going in a straight horizontal line e.g. from France to Russia (and the countries in between). It's common to say "travel across the United States" - i.e. from East Coast to West Coast. (Whereas "travel around the United States" just suggests going to lots of different states.) "Travel in Africa" - OK "Travel inside Africa" - no! "Travel around Africa" - good, see above "Travel across Africa" - only if going e.g. East to West (or West to East!) Hope this is helpful!