On Monday, Air Chief Marshal Angus Houston, who heads the agency co-ordinating the search, said it was time to deploy the Bluefin-21 to begin its search of the sea floor. It set off on its first mission on Monday night." After completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500m and its built in safety feature returned it to the surface," the Joint Agency Co-ordination Centre said in a statement.
Its built? What is it?
I may understand the meaning of safety feature, but it is very difficult to figure out what does "its built" mean.