Masjid al-Haram, or The Grand Mosque

Year Built: ongoing

Location: Mecca

At the heart of The Grand Mosque in Mecca lies the Ka'aba, or the house that Abraham built. The current structure covers 88.2 acres of land, and it is under another expansion project slated to finish in 2020. The structure can accommodate up to 4 million worshippers during the Hajj period, which constitutes one of the largest gatherings in the world.