...the genius responsible for this: Il Duomo. This is Cattedrale di Santa Maria del Fiore (Cathedral of Saint Mary of the Flower), also known as Il Duomo di Firenze. It is the iconic structure of Florence. Construction began in 1296 and it was completed, at the height of the Renaissance, in 1436 with the dome engineered by Brunelleschi (the pink and green travertine facing was added much later).
Brunelleschi was not a trained architect...he was a goldsmith by training. The story of the Duomo is so complex and fascinating that I cannot even begin do it justice here. Please do yourself a favor and Google it to read about its creation and construction; there is also a PBS documentary about it that is well worth seeing (available online).