[al:新概念英语（四）] [ar:MP3 同步字幕版（美音）] [ti:Exploring the Sea-Floor] [by:更多学习内容，请到yingyu.chazidian.com搜索“新概念”] [00:00.93]Lesson 30 [00:02.38]Exploring the sea-floor [00:09.48]How did people probably imagine the sea-floor before it was investigated? [00:17.45]Our knowledge of the oceans a hundred years ago was confined to the two-dimensional shape of the sea surface [00:24.90]and the hazards of navigation presented by the irregularities in depth of the shallow water close to the land. [00:33.53]The open sea was deep and mysterious, [00:36.86]and anyone who gave more than a passing thought to the bottom confines of the oceans probably assumed that the sea-bed was flat. [00:45.67]Sir James Clark Ross had obtained a sounding of over 2, 400 fathoms in 1839, [00:53.26]but it was not until 1869, when H.M.S. Porcupine was put at the disposal of the Royal Society for several cruises [01:03.38]that a series of deep soundings was obtained in the Atlantic and the first samples were collected by dredging the bottom. [01:11.84]Shortly after this the famous H.M.S. Challenger expedition established the study of the sea-floor [01:19.01]as a subject worthy of the most qualified physicists and geologists. [01:24.37]A burst of activity associated with the laying of submarine cables [01:29.62]soon confirmed the Challenger's observation that many parts of the ocean were two to three miles deep, [01:37.28]and the existence of underwater features of considerable magnitude. [01:42.74]Today, enough soundings are available to enable a relief map of the Atlantic to be drawn [01:49.02]and we know something of the great variety of the sea bed's topography. [01:54.78]Since the sea covers the greater part of the earth's surface, [01:58.56]it is quite reasonable to regard the sea floor as the basic form of the crust of the earth, [02:04.88]with superimposed upon it the continents, [02:08.74]together with the islands and other features of the oceans. [02:13.05]The continents form rugged tablelands which stand nearly three miles above the floor of the open ocean. [02:20.95]From the shore line, out to a distance which may be anywhere from a few miles to a few hundred miles, [02:28.63]runs the gentle slope of the continental shelf, geologically part of the continents. [02:35.45]The real dividing line between continents and oceans occurs at the foot of a steeper slope. [02:42.70]This continental slope usually starts at a place somewhere near the 100-fathom mark [02:48.91]and in the course of a few hundred miles reaches the true ocean floor at 2, 500-3, 500 fathoms. [02:59.08]The slope averages about 1 in 30, but contains steep, [03:03.49]probably vertical, cliffs, and gentle sediment-covered terraces, [03:08.14]and near its lower reaches there is a long tailing-off which is almost certainly the result of [03:14.36]material transported out to deep water after being eroded from the continental masses.