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In the early part of the 19th century it was commonly thought that marine life existed only in the upper levels of the oceans. In later years, animals were recovered from successively greater depths, so that by the time of the Challenger expedition in 1873, life seemed to exist almost everywhere in the ocean and on the sea floor. The remarkable abundance of the deep sea fascinated naturalists world-wide, and in 1877 the U.S. Coast Survey steamer Blake was outfitted for deep dredging and trawling, to be done in conjunction with sounding surveys of the eastern U.S. continental shelf and slope. With a waterline length of only 140 feet (43 m), the Blake was a rather small vessel for such arduous service, but these limitations were more than compensated for by Alexander Agassiz, who directed the first dredging survey, and Lieutenant Sigsbee, who commanded the vessel.