about Masjid al Aqsa

In Quran it is said > "Exalted is He who took His Servant by night from al-Masjid al-Haram to al-Masjid al- Aqsa, whose surroundings We have blessed, to show him of Our signs. Indeed, He is the Hearing, the Seeing." But as much as I know Masjid al Aqsa was build in the regain of Umar (pbuh) and before there was a garbage dump. I know masjid means a place of prostration regardless of religion, but if there was a garbage dump then how can it be a place of prostration? Can anyone please tell me about it?