1) The concept of augmented reality (AR) involves overlaying digital information and virtual objects onto the real world, enhancing the user's perception and interaction with the environment.

2) The Great Wall of China is a UNESCO World Heritage site and one of the most iconic man-made structures in the world. It stretches over 21,196 kilometers (13,171 miles) and was constructed to protect the Chinese empire from invasions during ancient times.

3) The Southern Ocean surrounds Antarctica and is the coldest and least-visited ocean.

4) The first modern Olympic marathon race was held at the 1896 Olympics in Athens and has since become one of the most iconic and challenging events in athletics.