Would an anvil dropped into the Mariana Trench descend at a uniform speed?

Beware falling anvils ... a jellyfish 3,700 metres deep in the Mariana Trench. Photograph: NOAA Office of Ocean Exploration and Research Photograph: NOAA

I’ve read that an anvil dropped overboard into the Mariana Trench would take an hour to reach the bottom. Would its speed of descent be constant all the way down?

Alan Paterson, London N8

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