Ships sinking over the horizon prove it, because that's what you expect to see on a flat earth. The horizon being lower than 180 degrees when measured by theodolite also proves it, because that's what you'd expect to see on a flat earth. Rotation of stars around two celestial poles also adds to the evidence, because that's what you'd expect to see on a flat earth. The sun rising and setting due east and west on the equinoxes proves it, because that's what you'd expect to see on a flat earth. Neutrinos passing down through the ground in a straight line and emerging elsewhere on the surface proves it, because that could only happen on a flat earth, couldn't it?
In fact, flat earth fits so well with absolutely everything we observe, it's surprising that you are asking where the proof is. 
No, you can not be this dumb. I sure hope this was sarcasm.
If earth was flat, ships would not sink below the horizon. That is what happens on a flat surface, things stay level.
Polaris and sigma octantis are the pole stars directly aligned with earth's rotational axis, north and south. If earth was flat, there would only be one polar star not two.
The sun would not set on a flat surface, if it did there would be night everywhere, it would fade and never go below the horizon.
I dont know anything about neutrinos exept that they travel at lightspeed, wierd things happen when you approach light speed.
My point is these are not proof, if anything they confirm round earth.