练习材料：
[Day 1776 2019-08-07]
Lesson 30-2 Exploring the sea-floor
A burst of activity associated with the laying of submarine cables soon confirmed
the challenger's observation that many parts of the ocean were two to three miles deep,
and the existence of underwater features of considerable magnitude.
Today, enough soundings are available to enable a relief map of the Atlantic to be
drawn and we know something of the great variety of the sea bed's topography. Since
the sea covers the greater part of the earth's surface, it is quite reasonable to regard the
sea floor as the basic form of the crust of the earth, with, superimposed upon it the
continents, together with the islands and other features of the oceans. The continents
form rugged tablelands which stand nearly three miles above the floor of the open
ocean.
任务配置：L0+L1+L4
知识笔记：
submarine cables 网络 海底电缆;
of considerable magnitude 相当大的
magnitude n. 巨大; 重大; 重要性; 星等; 星的亮度; 震级;
relief n. (不快过后的) 宽慰，轻松; 解脱; (焦虑、痛苦等的) 减轻，消除，缓和; (给灾区或交战地区人民提供的) 救济，救援物品;
relief map n. (用不同颜色表示地势高低的) 地形图，地势图，地貌图;
topography n. 地形; 地貌; 地势; 地形学;
superimposed v. 使(图像甲)叠映在(图像乙)上; 使重叠; 使叠加; 使附加于;
rugged adj. 崎岖的; 凹凸不平的; 多岩石的; 强健而富有魅力的; 粗犷的; 坚强的; 坚毅的;
tablelands 网络 高原;
练习感悟：
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