Choose the correct words to complete the summary about blue holes.Blue holes are caves that are1___________________ (underwater / high up in the mountains). They were formed millions of years ago when2___________________ (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 very3___________________ (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 about4___________________ (ancient life forms / how the oceans were formed).