Is A Rainbow A full circle that Also Goes underneath the ground?

Yes that may be so. But what I do know for sure a rainbow is a Symbol that the Father Our Creator showing a symbol that he will not Flood the earth again. So we know the next time in end of days it won’t be a flood it be something even more worse to destroy the earth and everything and everything one in it. That is what the rainbow is for, only a few will make it into the New Kingdom.

Rainbow is formed on the principle of refraction of light. That is bending of light when it passes through a condensed water vapour. That it the reason it appears bending but not in the real sense. It is normally straight because light rays goes in a straight lines.

From what I’ve heard over the years,  you need light and water droplets for a rainbow, so as far as these 2 elements combine the rainbow will be and yes they are circular , though we only ever see half circle because of the horizon.