This is not an ordinary Manhole Cover, this is Suatac:  Service Utility Assembly Totally Adjustable Cover
 

Technical Details - 24" Cover

Adjustable Manhole cover Assembly, height and slope by the millimeter. With frost wings and slip-joint which make it stay level with the road, all year round.

A load test report was completed in Germany in July 2008 and was able to withstand over 40 tons of pressure.

A: Final slope adjustment, mating semi-spherical surfaces
B: Rough slope adjustment, mating semi-spherical surfaces