英語聽力：自然百科 佛羅里達(dá)州的溫泉（二）

The divers started in the ocean, but they are now within solid bedrock, 150 feet beneath someone's living room. They are now swimming in the water supply for the town above. Here they find, as they had hoped, life in the pitch darkness. These are hydroids, small filter-feeding organisms, a species entirely new to science. Around the hydroids are mussels. Neither of them are found in the springs above. Both are anchored to the rock and live on food that drifts by. But there is no apparent sign of food and no energy to create it. In almost every living system, the sun provides fuel for all life. But there is no sunlight here, how these organisms kept alive?

With such exciting discoveries, the team decides to press on, but time and air are finite. Their air is consumed quickly at this depth and each minute they stay will increase the time they must spend decompressing before they can leave the water. The slightest turbulence stirs up sediment which can render visibility from hundreds of feet to nearly zero. Then, 3,000 feet into the springs, they make an extraordinary discovery--a strange layer of water.

The layer below is saltwater but clear, fresh water flows above. Where the layers meet, there is a thin, cloudy substance that they have never seen before. Later analysis reveals it to be bacteria. It lives on a microscopic horizon between fresh and saltwater. It is this bateria which provides the basis for all life in the cave. The bacteria metabolizes sulfur from the water and multiplies. In turn, the bacteria becomes food for the mussels and hydroids downstream. In this cave, lowly bacteria takes the place of sunlight and starts the chain of life.

The divers christen the cave of their discovery "the dragon's lair" in honor of its unique smoky resident. It's a shock to realize that there is so much life within the town's water supply. But it's probably an indicator of healthy water. If the life here vanishes, maybe then we should worry about the quality of our drinking water.