It is the Domo in Florence, Italy. It is so big that you can’t take one picture to get the whole cathedral. I know I have tried.
It was built by Brunelleschi
he built the cathedral of Florence
Arnolfo di Cambio and Filippo Brunelleschi were the two architects that built the Florence Cathedral.
Filippo Brunelleschi
It was Brunelleschi. He built the dome of the Basilica di Santa Maria del Fiore, Florence's cathedral.
The Dome!
The dome in the Cathedral of Florence was designed by Arnolfo di Cambio. It was engineered by Filippo Brunelleschi and completed in 1436.
It is 114 metres tall.
The Cathedral of Florence
You might be thinking of the Pantheon, however the Pantheon was more hemispherical and would not have worked on the scale of Brunelleschi's dome so he had to use newer Gothic methods.
Filippo Brunelleschi
Brunelleschi's Dome is a round structure. If you do an internet search for "Santa Maria del Fiori" you will see pictures of the Florence cathedral and Brunelleschi's famous dome.