Complex sentence: By around 1980, much evidence had been accumulated to form conjectures about elliptic curves, the main reason to believe that these various conjectures about elliptic curves were true lay not in the numerical confirmations, but in a remarkably coherent and attractive mathematical picture that they presented.
Split into simpler sentences: By around 1980, much evidence had been accumulated to form conjectures about elliptic curves. The main reason to believe that these various conjectures about elliptic curves were true lay not in the numerical confirmations, but in a remarkably coherent and attractive mathematical picture that they presented.