Passage 5
学霸导读 古老的猛犸象牙价值连城，但是开采冻土层的象牙会破坏生态平衡，影响人与野生动物的生存环境，我们应该禁止这一行为。
Because permafrost （ 永冻土层 ） is so good at preserving animal tissue （ 细胞组织 ）， there are thousands of mammoth tusks （ 猛犸象牙 ） trapped u 1 . Please visit Siberia to unearth these tusks—90 percent of which end up in China. Carved ivory ornaments （ 装饰 ） are a status （ 地位 ） symbol， but elephant ivory is banned （ 禁止 ） in China.
In some places， mammoth ivory is c 2 as an ethical （ 合乎道德的 ） source of ivory， so the demand for it is sky high. A 65-kilogram tusk can be sold for 23，000 pounds.
The number of tusks still encased （ 埋在 ） in permafrost seems to have diminished signif icantly， and hunters are struggling to f ind v 3 prehistoric tusks now that they are in low supply.
But not only is the scientif ic community （ 科学界 ） losing potential discovery to the ivory trade； the permafrost itself is also suffering. Mammoth hunters blast （ 爆破 ） the ice and excavate cavernous （ 像洞穴的 ） tunnels， c 4 unnecessary damage. Sludge （ 烂泥 ） from the melted ground also runs into nearby rivers， making the water dirty and more dangerous.
Permafrost p 5 the ground from absorbing water， creating wetlands and lakes that normal rainfall couldn’t sustain. People and wildlife rely on these environments to survive， and the loss of permafrost would have an enormous impact. If subterranean （ 地下的 ） ice were to melt， the land would change drastically （ 大幅度地 ）. Some areas would dry out， while others would get w 6 . The chemical make-up of the water would change and the ground would become irregular （ 不规则的 ）. The new landscape could alter the course of rivers， affecting wildlife.
U 7 ， permafrost is beginning to melt owing to our changing climate. The local polar release of methane （ 甲烷 ） alone could double in the next 100 years.
1.__________2.__________3.__________4.__________5.__________6.__________7.__________
高频词汇
unearth /ʌn'ɜːθ/ v. 发掘，掘出，使出土
end up 最后来到
diminish /dɪ'mɪnɪʃ/ v. 减少，减小，降低
struggle /'strʌgl/ v. 奋斗，努力，拼搏
excavate /'ekskəveɪt/ v. 发掘；挖出
sustain /sə'steɪn/ v. 保持，维持，使持续，使继续
make-up /'meɪkʌp/ n. 组成，构成
alter /'ɒltər/ v. （通常指轻微地） 改动，修改；改变