I remember reading about how the built the Pantheon in Rome. They had it filled with sand inside to support the structures when being built. And how did they get the sand out? Easy, they had put coins in the sand an a little treasure beneath the sand for the Roman rabble of the city to get. The poor of Rome emptied the temple in no time of the sand.
Btw. after nearly 2 000 years (built in 126 AD), the Pantheon's dome is still the world's largest unreinforced concrete dome:

