The real mystery is how Sephiroth was 'produced' by Professor Gast. Seeing as how Hojo injecting Jenova Cells into him just turns him into something completely pointless and he's dead after 2 Ultimas and a D.Blow. Lucrecia does not die either, as the Jenova Cells spread from her womb into her body, and they grew with her.

The only person who nears Sephiroth's greatness is Cloud Strife, a young boy who is killed, then resurrected by Hojo using a style to 'produce' adopted from Professor Gast.

Back to the point, how does Gast produce Sephiroth?

Suggestions?

Cloud never died

It's not a mystery how it happened. Hojo and Lucrecia had they'er baby injected with Jenova Cells whilst still in the womb to create a super-human baby, Sephiroth. All this was done under the supervision of Gast who was head of the Jenova Project so he takes most credit for it.