参考答案：under the sea

解析：[听力原文]1-20 Earthquakes often happen near volcanoes, but this is not always true. The centres of some are under the sea. The bottom of the sea suddenly moves. The powerful forces inside the earth break the rocks. The coast is shaken and great waves appear. These waves travel long distances and rush over the land when they reach it. They are strong enough to break down houses and other buildings; sometimes they break more buildings than the earthquake itself. Very often fires followed the most serious earthquakes. In 1906, the great earthquake at San Francisco broke the gas-pipes. The gas escaped, and soon large numbers of fires were burning in the city. The water-pipes were also shaken and broken; so it was not possible to put the fires out. There was no water. The Tokyo earthquake of 1923 happened just before the middle of the day. People were cooking meals on their fires at that time. When the ground shook, the fires shook too. Hot materials were thrown on the different parts of the houses, some of which were made of wood. Soon 134 fires were burning in the city. What kind of building stands up best in an earthquake A building with concrete walls is perhaps the best. A steel frame will make it even stronger. The frame holds the different parts together, and the walls do not easily fall. There is less chance of fire because concrete and steel do not burn. The Americans carefully studied the results of the earthquake in San Francisco, and they are convinced that this kind of building is the safest.[解题技巧] 完形填空是对应试者听的能力、拼写能力、记笔记能力和书面表达能力的综合测试。它要求应试者不仅听懂内容，熟练而准确地拼写单词，更重要的是将一些短语、句子结构正确理解并表达出来，是对英语综合运用能力的考查。 在答题时注意： 1．预览文章，找出线索。文章的话题规定了用词范围；利用词汇的同现和复现关系；根据上下文推测词汇的运用。 2．听写结合，双管齐下。听音贯穿着期待、预知、分析、综合、推理和判断等一系列过程。听的同时应快速记下关键词；而在记笔记时，又要能有效、专注地去听，获取全文信息。 3．提高记笔记的效率。首先，使用缩略语；字母较少的单词，可完整写出该词；字母较多的单词只写该词的几个字母。其次，有选择地记笔记，应重点记下句中的中心词和实词。

[分析]： 介词短语。 本题考查上下文的呼应。通过上下文可知下句是对本句的进一步解释，其中the bottom of the sea应与本题空格处意思相象。该词组在句中作表语。