Here is a description from wikipedia (don't worry, they are the same facts our tour guide told us):
"Built on reclaimed land, almost half of the surface of the mosque lies over the Atlantic water. This was inspired by the verse of the Qur'an that states "the throne of God was built on water." Part of the floor of this facility is glass so worshippers can kneel directly over the sea; above, spotlights shine at night from the top of the minaret toward Mecca. These features were specifically requested by King Hassan II, who declared, "I want to build this mosque on the water, because God's throne is on the water. Therefore, the faithful who go there to pray, to praise the Creator on firm soil, can contemplate God's sky and ocean." It also includes a number of modern touches: it was built to withstand earthquakes and has a heated floor, electric doors, and a sliding roof."
Location:Calle de Arlabán,Madrid,Spain