Unfortunately not every question can be answered. Some mysteries were never meant to be solved and these are just a few that will most likely remain unsolved forever.
Beihai is a city in China which is now known as “The City of the Dead.” It was originally built as a place for the upper class to live but now no one lives there and it's a ghost town.
You've probably seen people building an SOS on the beach in TV shows or maybe a movie and thought something like that would never work in real life. Think again. These people got stranded on an island, built a giant SOS and it ended up getting them rescued. When it doubt, build an SOS.