Every year millions of Muslims travel to Mecca for the hajj, one of the five pillars of Islam. Muslims travel to Islam’s most sacred mosque, al-Masjid al-Haram, during the six-day pilgrimage. Mecca is thought to be the place where Ishmael and his mother Hagar were provided with a spring of water in the desert. As it is the most sacred place in Islam, non-Muslims are forbidden from entering.

In the center of the mosque, there is a black, box-shaped building, which has sparked questions from those unfamiliar with the Islamic culture. Here is everything to know about the most sacred space in the Muslim world.

What is the cube-shaped structure called?

It is called the Kaaba, or "the cube."

Is there anything inside?

The Kaaba is built around a sacred black stone, a meteorite that Muslims believe was placed by Abraham and Ishmael in a corner of the Kaaba, a symbol of God's covenant with Abraham and Ishmael and, by extension, with the Muslim community itself. It is embedded in the eastern corner of the Kaaba.

What is the historical significance of the Kaaba?

Muslims believe the Kaaba was originally built by Abraham and Ishmael, but the site was re-dedicated by Muhammad and has been reconstructed since. When Muslims pray, wherever they are, they turn toward the Kaaba, and during the hajj, pilgrims walk counterclockwise around it seven times.

What is the Kaaba made of?

The Kaaba is made of granite from the hills near Mecca. The structure is 50 feet high (15.24 meters), 35 feet (10.67 meters) wide and 40 feet long (12.19 meters) long. Inside the Kaaba, the floor is made of marble and limestone. The interior walls, measuring 43 feet (13 meters) by 30 feet (9 meters), are clad with tiled, white marble halfway to the roof, with darker trimmings along the floor. It is covered by a black silk cloth decorated with gold-embroidered calligraphy. This cloth is known as the kiswa, and it is replaced yearly.