The Moon is the Earth's only natural satellite. It travels around the Earth about once every month. The Moon does not make its own light; we see it because it reflects sunlight. As it moves, we see different amounts of its lit side, so it seems to change shape. These changes are called phases. The Moon's gravity also pulls on the oceans, and this causes the tides, the rise and fall of the sea level each day. In 1969, Neil Armstrong became the first person to walk on the Moon, and twelve people in total have walked there. The surface is covered with dust and craters, and there is no air. The temperature changes greatly between day and night. Scientists continue to study the Moon, and several countries are planning new missions.

Question: In the sentence 'twelve people in total have walked there', what does 'there' refer to?