Rub' al Khali is a location in 80 Days. It is located in the Arabian Peninsula and is the largest sand desert in the world.
You will meet the Court inventor al-Talib here, who believes you are villainous spies and will test her latest invention on Fogg and Passepartout. Al-Talib's Invention is a transporter that will send you different places depending on which way you fall as you pass out in the transporter.
The only way in is with the Bedouin Expedition.