The Volkswagen Audi Group (VAG) mounts a golf ball like belly pans like
LONG FIBRE PP COMPOUNDS DRIVE AUTOMOTIVE INNOVATION or
VWVortex.com - 3.2 belly pan on the 2.0t
The drag reduction is because aerodynamics of turbulence and laminar flows thanks to wake reduction.
These belly pans are used in my car underbody Seat Leon but also in Volkswagen Golf (GTI, Rabbit,...), Skoda Octavia and Audi A3 (technologycally the same car).