PETS

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B

Nearly 100 feet under the Yucatán Peninsula1 in southern Mexico, cave divers are mapping the world’s longest underground river. More importantly, they are using this map to explain the mysteries of the ecosystem that may be destroyed before it is fully understood.

The Peninsula is rich in human history. It is proven by all the temples and pyramids built by the Maya in ancient times. A system of rivers whose formation began more than 100 million years ago runs underground. This system was developed when the Peninsula lay under a low sea.

Over the ice ages, sea levels dropped about 300 feet, exposing the stone platform that makes up the Peninsula. Over time, small streams of carbonic acid2 created the caves. When sea levels began to rise with the last ice age 18,000 years ago, the once dry caves began to fill with water, a process that continued until about 1,000 years ago. In fact, these river systems provide all of the Peninsula’s fresh water.

By far the largest underground river system is called Ox Bel Ha. Its complex3 passageways, about 200 miles, surrounds the Cancún city, the late classic Maya coastal trading center of Tulúm .

Since 1998, an international team of divers has been charting Ox Bel Ha. Some dives last more than 12 hours in the water, the time necessary to reach Ox Bel Ha’s deepest places, map them and safely return to the surface.

Till now, the team has charted more than 60 miles of caves underwater, and three freshwater passageways that are connected to the Ox Bel Ha system. More excitingly, the cave divers found human bones from the Maya period. Scientists believed that the caves played an important role in Maya religion: they were regarded as entrances to the underworld, a mysterious place of gods and ancestors.

Notes:

1. 尤卡坦半岛(古玛雅文化的摇篮之一)

2. carbonic acid碳酸

3. complex adj.复杂的

1. What is the passage mainly about?

A. The temples and pyramids built by the Maya.

B. Divers discover Maya relics in caves that became rivers.

C. The world’s longest underground river in Peninsula.

D. The largest underground river system, Ox Bel Ha.

2. According to the passage, which of the following statements about time and event is FALSE?

A. About 1,000 years ago, the dry caves began to fill with water.

B. More than 100 million years ago, the Peninsula lay under a low sea.

C. For the past 1,000 years, a river system ran underground under the Peninsula.

D. Since 1998, the largest underground river system has been charted.

3. What we can know from the passage about the mapping are the following EXCEPT that .

A. it is very useful for protecting the ecosystem

B. it is a long and hard course

C. it is dangerous for the divers

D. it has achieved many fruits

参考答案：BAC

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