Herod built the foundation by filling in valleys. He expanded the Temple Mount with arches and concrete.
Jesus and the Temple
"So Herod took away the old foundations, and laid others, and erected the temple upon them, being in length a hundred cubits, and in height twenty additional cubits, which upon the sinking of their foundations, fell down: and this part it was that we resolved to raise again in the days of Nero."– Josephus, Antiquities 15.11.3 (391)