Today, the Kaaba is kept closed during the hajj because of the overwhelming number of people, but those who visit the Kaaba during other times of the year are sometimes allowed to go inside. It's quite beautiful: The walls are white marble on the lower half and green cloth on the upper half.

What's inside the Kaaba?

The interior contains nothing but the three pillars supporting the roof and a number of suspended silver and gold lamps. During most of the year the Kaaba is covered with an enormous cloth of black brocade, the kiswah. The Kaaba surrounded by pilgrims during the hajj, Mecca, Saudi Arabia.

Who can visit the Kaaba?

All Muslims aspire to undertake the hajj, or the annual pilgrimage, to the Kaaba once in their lives if they are able. Prayer five times a day and the hajj are two of the five pillars of Islam, the most fundamental principles of the faith.