The correct answer to this open question is the following.
We are referring to one of the most beautiful buildings constructed by ancient Rome: the Pantheon; an impressive Roman building with a dome that has a hole at the top of the dome.
The hole served to the following purpose. Originally, it was the only source of light. The Sun was the source of light and its rays entered the building through the hole. Today, the Pantheon is a Catholic church and is one of the most famous touristic attractions in Rome.