短语讲解：

1. It begins at the bottom of the Pacific Ocean, and rises more than 17 kilometers from the ocean floor.

at the bottom of 在…的底部
例句：The book I want is (right) at the bottom of the pile.
我要的那本书（就）在这堆书底下。

2. In fact, NASA uses the top of Mauna Loa to simulate life on Mars for researchers.

in fact 事实上; 实际上
例句：They were in fact casual.
事实上，他们确实不修边幅。

3. A 20-minute walk through a forest leads to a cave-like tube.

lead to 导致; 把…带到; 领到
例句：That may lead to a serious split in our class.
那可能导致我们班的严重分裂。