The Great Blue Hole, thousands of years old. They are underwater sinkholes also called Vertical Caves. There are many different blue holes located around the world, but the best known is 'The Great Blue Hole" of BELIZE.
Looking down the Inverted Tower or Initiation well of Quinta da Regaleira, Sintra, Portugal. The owner wanted to develop the palace and its grounds as a gathering of symbols that would reflect his interests and ideologies.