yea i was thinking about the problem with laser not reaching long distances through water, but if you just used a rope that doesnt float instead of it than the fact that is sinks proves nothing so it hadf me thinking for Quite a while until i finnaly found a better solution (still more theoretical than practical but i've always been a man of theory). you use a "rope" that has the exact same density as sea water :D that way it neither floats nor sinks, it just kinda hangs around in the middle of the water. of course as you go deeper water gets a bit more dense so the rope would eventually bend over a deeper layer of water but that doesn't really matter. as long as one rope stays on the surface and the other starts to slowly "cut" into the water, sinking deeper and deeper you have proven that the surface is curved (though not necessarly spherical it could be an oblate (or whatever Junior called it)).