[al:新概念英语（四）] [ar:MP3 同步字幕版（英音）] [ti:Exploring the Sea-Floor] [by:更多学习内容，请到chazidian.com搜索“新概念”] [00:01.52]Lesson 30 [00:03.24]Exploring the sea-floor [00:11.90]How did people probably imagine the sea-floor before it was investigated? [00:20.08]Our knowledge of the oceans a hundred years ago [00:23.21]was confined to the two-dimensional shape of the sea surface [00:27.87]and the hazards of navigation presented by the irregularities in depth [00:32.65]of the shallow water close to the land. [00:36.48]The open sea was deep and mysterious, [00:40.10]and anyone who gave more than a passing thought to the bottom confines of the oceans [00:45.51]probably assumed that the sea bed was flat. [00:49.62]Sir James Clark Ross had obtained a sounding of over 2, 400 fathoms in 1839, [00:57.40]but it was not until 1869, [01:00.25]when H.M.S.Porcupine was put at the disposal of the Royal Society for several cruises [01:07.71]that a series of deep soundings was obtained in the Atlantic [01:12.46]and the first samples were collected by dredging the bottom. [01:17.08]Shortly after this the famous H.M.S.Challenger expedition [01:21.56]established the study of the sea-floor [01:24.00]as a subject worthy of the most qualified physicists and geologists. [01:29.67]A burst of activity associated with the laying of submarine cables [01:34.51]soon confirmed the Challenger's observation [01:37.28]that many parts of the ocean were two to three miles deep, [01:41.78]and the existence of underwater features of considerable magnitude. [01:47.04]Today, enough soundings are available to enable a relief map of the Atlantic to be drawn [01:53.68]and we know something of the great variety of the sea bed's topography. [01:59.24]Since the sea covers the greater part of the earth's surface, [02:02.97]it is quite reasonable to regard the sea floor [02:05.79]as the basic form of the crust of the earth, [02:09.05]with superimposed upon it the continents, [02:12.47]together with the islands and other features of the oceans. [02:16.79]The continents form rugged tablelands [02:20.34]which stand nearly three miles above the floor of the open ocean. [02:25.58]From the shore line, [02:26.87]out to a distance which may be anywhere from a few miles to a few hundred miles, [02:32.53]runs the gentle slope of the continental shelf, geologically part of the continents. [02:39.97]The real dividing line between continents and oceans [02:43.41]occurs at the foot of a steeper slope. [02:47.37]This continental slope usually starts at a place somewhere near the 100-fathom mark [02:53.81]and in the course of a few hundred miles [02:56.35]reaches the true ocean floor at 2, 500-3, 500 fathoms. [03:04.26]The slope averages about 1 in 30, but contains steep, [03:09.07]probably vertical, cliffs, and gentle sediment-covered terraces, [03:15.38]and near its lower reaches there is a long tailing-off [03:18.92]which is almost certainly the result of [03:21.38]material transported out to deep water after being eroded from the continental masses.