After the clash of the Titans the losers (giants also) were made into slaves. Women raped then eaten, so were the children. The slaves hands were removed so they couldn't revolt....but they could carry stone easy.

There are 1000s of pyramids under the oceans. Giants could build one in a few weeks.

The pyramids are entrances and exits into hell. They needed a quick way underground in the event the arch angels came, a meteor came, a flood, etc.