Hello, everybody!
Thank everybody for helping me. I'm really grateful for it.
I know that flaw's synonym is 'fault' when 'flaw' means mistake. In my opinion, 'flaw' in this sentence should be 'a deep crack, especially in rock or earth', so I think 'gap, cleft,fissure' are all correct, except 'fault', but the question is which one is the most suitable answer among them. I'm still confused.:-?
I posted some more context to help everybody understand it. I really wish that everybody could still keep an eye on this thread, because I haven't found out the answer so far. Thanks!!
Here it is:
Cave exploration， or pot-holing， as it has come to be known， is a relatively new sport. Perhaps it is the desire for solitude or the chance of making an unexpected discovery that lures people down to the depths of the earth. It is impossible to give a satisfactory explanation for a pot-holer's motives. For him， caves have the same peculiar fascination which high mountains have for the climber. They arouse instincts which can only be dimly understood. Exploring really deep caves is not a task for the Sunday afternoon rambler. Such undertakings require the precise planning and foresight of military operations. It can take as long as eight days to rig up rope ladders and to establish supply bases before a descent can be made into a very deep cave. Precautions of this sort are necessary， for it is impossible to foretell the exact nature of the difficulties which will confront the pot-holer. The deepest known cave in the world is the Gouffre Berger near Grenoble. It extends to a depth of 3，723 feet. This immense chasm has been formed by an underground stream which has tunneled a course through
a flaw in the rocks. The entrance to the cave is on a plateau in the Dauphine Alps. As it is only six feet across， it is barely noticeable. The cave might never have been discovered has not the entrance been spotted by the distinguished French pot-holer， Berger. Since its discovery， it has become a sort of potholers' Everest. Though a number of descents have been made， much of it still remains to be explored. A team of pot-holers recently went down the Gouffre Berger. After entering the narrow gap on the plateau， they climbed down the steep sides of the cave until they came to narrow corridor. They had to edge their way along this， sometimes wading across shallow streams， or swimming across deep pools. Suddenly they came to a waterfall which dropped into an underground lake at the bottom of the cave. They plunged into the lake， and after loading their gear on an inflatable rubber dinghy， let the current carry them to the other side. To protect themselves from the icy water， they had to wear special rubber suits. At the far end of the lake， they came to huge piles of rubble which had been washed up by the water. In this part of the cave， they could hear an insistent booming sound which they found was caused by a small waterspout shooting down into a pool from the roof of the cave. Squeezing through a cleft in the rocks， the pot-holers arrived at an enormous cavern， the size of a huge concert hall. After switching on powerful arc lights， they saw great stalagmites —— some of them over forty feet high —— rising up like tree-trunks to meet the stalactites suspended from the roof. Round about， piles of limestone glistened in all the colours of the rainbow. In the eerie silence of the cavern， the only sound that could be heard was made by water which dripped continuously from the high dome above them.
