The floor of the Thames Tunnel in 1856 was 34.9 feet below the floor of the River Thames (i.e., 65.9 - 31.0).  For a more extended view, go to the 1856 Waterworks map by clicking here and move to cell B4.

Source: Mylne RW. Map of the Contours of London and Its Environs,  showing the Districts and Areas supplied  by the Nine Metropolitan Water  Companies, Published for the Author by Edward Stanford, Charing Cross,  London.  Engraved and Printed from Stone by Waterlow and Sons, 1856.
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