Ablueholeisafloodedseacavewithaholethatopensupattheland'ssurface．Thesecavesystemsformincarbonaterock，oftenonislands．Someblueholeshaveveryspecialrockformationsandwaterchemistry．Farbelowsealevel，theycont

Question

A blue hole is a flooded sea cave with a hole that opens up at the land's surface．These cave systems form in carbonate rock，often on islands．Some blue holes have very special rock formations and water chemistry．Far below sea level，they contain some of the harshest environments on Earth，with no oxygen and no light．Yet these areas are filled with life forms that have adapted to the extreme conditions．
Information gathered from these blue holes is helping scientists to increase their understanding of biology，archaeology and geology．But exploring these blue hole environments brings danger as well as discovery．
Blue holes get their name from the color some have when seen from the air．The color is usually a reflection of the sky on the water．But not all of these cave systems have blue surfaces．Some contain dark or muddy water．
Blue holes are the result of erosion in which water breaks down rock．Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass．These holes were later filled with sea water as the sea level changed．The rising and falling of sea levels and the mixture of salt and fresh water further wore away at these cave formations．Blue holes are vertical caves．But they can also have horizontal cave formations that may be hundreds of meters long．
These flooded cave systems can be found in the ocean，or they can be found inland．Ocean caves are affected by tides，so they always have water movement．But blue holes on land are very still．They have several layers of water，chemicals，and bacteria．The top layer of fresh water comes from rainfall．This layer acts like a cap on top of the layered mixture，and keeps out oxygen from the atmosphere．The fresh water floats on a denser layer of saltwater．Underneath this is a layer of poisonous hydrogen sulfide，produced by bacteria living in the water．Underneath this layer is anoxic seawater--water that does not contain any oxygen．
Underwater caves are probably one of the least explored ecosystems on the planet．One reason blue holes have not been fully explored is that they can be extremely dangerous．There are many safety rules that divers must follow to help ensure their survival．
First，divers must have training and experience to swim in these caves．Those who explore a cave for the first time must establish a thin rope called a guideline．This line helps them to safely enter and bring more than one set of breathing equipment in case one device fails．Meanwhile，they must pay careful attention to their air supply．The rule is to use a third of their air to enter the cave，a third to exit，and a third for emergencies．
    For most explorers，though，the possibility of discovery in these cave environments makes the experience worth the risk．You can jump into what looks like an insignificant little hole in the ground，and come out with information that's of value to many different disciplines，Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago，before oxygen existed on our planet．，as well as similar to space．
50．According to the Passage，blue holes___．
A．are now a popular choice for tourists
B．are flooded caves found mostly on islands
C．have appeared as rocks pile up year by year
D．can give off bright blue light deep in the caves
51．It can be inferred front the passage that___．
A．quantities of advanced life forms exist in blue boles
B．blue holes are not fully explored due to their low value
C．blue holes are vertical caves because of strong currents
D．the chemicals in the rain lead to the formation of blue caves
52．The fifth paragraph mainly talks about___．
A．two different types of blue holes    
B．how a blue hole comes into being
C．why scientists are interested in blue holes       
D．rules for divers exploring blue holes
53．When exploring blue holes，divers must___．
A．have a thin rope as a guideline anytime           
B．wear glasses in case of bright blue light
C．swim as quickly as possible to save time          
D．learn to make good use of their air supply
54．The exploration of blue holes can help us___．
A．discover moue energy resources  
B．understand the origin of life on Farth
C．learn about the movements of seawater
D．get ready for the exploration of the ocean．

Answer 1

BDADB
50   B   细节题． 根据文章第一 段A blue hole is a flooded sea cave with a hole that opens up at the land's surface． 可知蓝洞是在岛上被淹没的洞穴，故选B．
51    D   推理判断题．  根据文章第四 段 Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass．可知雨中的化学物质导致了蓝洞的形成，故选D．
52   A  细节题． 根据文章第 五段 These flooded cave systems can be found in the ocean，or they can be found inland．可知第五段讲述的是蓝洞有两种不同的类型，故选A．
53   D   细节题． 根据文章倒数第 二段Meanwhile，they must pay careful attention to their air supply 可知潜水员在探索蓝洞的时候，必须小心使用空气供应，故选D．
54   B   细节题． 根据文章最后一段Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago，before oxygen existed on our planet．，as well as similar to space． 可知探索蓝洞对于理解生命起源是有帮助的，故选B．