Eratosthenes figured it out. I don't really know what you mean by this question, though.
It was well before man explored that it was a known fact that the earth was round. The Ancient Greeks had that figured out thousands of years before the first ship hit the water.
Besides Earth, mars is the most explored planet.
Earth
PORN
none
none
Hubble has never "explored" Neptune. It is a telescope in orbit round Earth and from Earth it has "imaged" Neptune. It has done this several times eg 1996 and 1998 (there may be more times!).
umm: "The Earth is round"? ------------------------------------------------ The Earth is not round - it is an oblate spheroid. ---------------------------------------------------------------------------- A (slightly) oblate spheroid seems pretty "round" to me.
Pythagoras stated the earth was round
The moon rotates round the Earth.
the earth is not perfecly round
It is round!