Blue holes are caves that are (1) underwater / high up in the mountains. They were formed millions of years ago when (2) huge earthquakes happened / the land above them fell in and water entered the space. Blue holes are located on land or in the sea. Many of them are very deep and the water inside the holes is usually very (3) rough / calm. Although exploring blue holes is dangerous, many scientists still risk their lives to dive into them. They think that blue holes can tell us a lot about (4) ancient life forms / how the oceans were formed.