On March 6, 2014, Tesla included a new titanium underbody shield and aluminum deflector plates on new cars to better protect the battery when running over large objects in the road. Tesla also offers this retro-fit to all older cars for free. This does add about 7.5 lbs to the car but has a negligible effect to the range. Contact your service center to check if your car has the shield or to arrange the installation.
Check out this short video of the Underbody Shield Installation from Consumer Reports.
Tesla Blog: Titanium Underbody Shield