The oldest and largest of the Giza Pyramids is that of the IV Dynasty pharaoh Khufu – better known as Cheops – who probably reigned 2589–2566 BC. It originally stood 140m high and measured 230m along its base, but the removal of its casing stones has reduced these dimensions by three metres. The pyramid is estimated to weigh six million tons and contain over 2,300,000 blocks whose average weight is 2.5 tons (though some weigh almost 15 tons). This gigantic mass actually ensures its stability, since most of the stress is transmitted inwards towards its central core, or downwards into the underlying bedrock. It is thought to contain three main chambers: one in the bedrock and two in the superstructure. By the time archeologists got here, their contents had long since been looted, and the only object left in situ was Khufu’s sarcophagus. In 1993, a German team using a robot probe accidentally discovered a door with handles supposedly enclosing a fourth chamber, apparently never plundered by thieves, which might contain the mummy and treasures of Cheops himself. Another robot, sent down in 2002, pushed a camera through a hole drilled in the door to reveal another, similar door behind it. Further probes have been sent down since then, but no new chambers have been discovered.