Q: Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Scientists in Mexico have just begun a new study of one of the world's biggest pyramids: the Pyramid of the Sun, north of Mexico City. They're putting lots of small, high-tech machines under the pyramid to try to unlock some of its secrets. For thousands of years, people have tried to uncover the secrets of the pyramids. The people who built the pyramids made lots of secret doors and rooms to stop robbers from finding the treasures inside. However, there always have been some smart thieves in history. Now, almost all of Egypt's pyramids have been robbed, including the Great Pyramid of Khufu, which is the largest pyramid in the world. It is the only one of the ancient wonders of the world that is still standing. In AD 820, an Arab king named Abdullah AL Manum got a group of workers to dig their way into the Great Pyramid and have a look. Inside the pyramid, they found three rooms --- the Queen's Chamber, the Grand Gallery and the King's Chamber. But to their surprise, the men didn't find the treasures they wanted. The Queen's and King's Chambers were both empty! Where were the King's mummy and his treasures? Had someone already taken them away? The huge stone doors at the pyramid's entrance were still closed when AL Manum's men went inside. How had the thieves got in and out? Since then, many people have gone inside the Great Pyramid to have a look or to try to take things. But still, no other chambers or walkways have been found. In 2002, an American team made the most recent visit to the Great Pyramid. Scientists sent a robot into the pyramid, but they only found a mysterious locked stone door.
A:
What is the purpose of making some secret doors to the pyramids?