It is true the fundamental origin of gravity IS just a theory, just because it is proportional to mass does not mean mass is the cause of gravitational force. This is still a mystery to science. HOWEVER 9.81m/s^2 IS the acceleration of objects due to earths gravity. I personally have measured and confirmed this well known force several times via experimentation along with countless others throughout history and 9.81m/s^2 stands the test of time. Finnian is correct in both his explanations of gravity pulling water and the hiding of classified locations with digital fakes.

Here is a thought experiment: take a theoretical string that is 1000 miles long, lay it against any curved surface (including the earth) and drive along that string. how many miles does the odometer read at the end of the string?

Every curved surface if looked at close enough is a straight line.
Every flat and smooth surface if looked at close enough is a rough surface.
(I look through microscopes every day and see these facts with my own 2 eyes)

I was also a sailor in the USCG before becoming an engineer and sailed all over this globe. I have looked through binoculars and observed ships appearing and disappearing over the horizon.