In a Master's thesis in 1985 and a subsequent paper published in 1992, the author discovered that the universal separable metric space (up to isometry) discovered by Urysohn in 1925 has a uniquely determined linear closure (up to linear isometry) when isometrically embedded in a Banach space so as to include the zero of the Banach space. The proof of this result is given in this note and the current status of some related questions is discussed.
Holmes, Randall. (2008). "The Urysohn Space Embeds in Banach Spaces in Just One Way". Topology and its Applications, 155(14), 1479-1482. http://dx.doi.org/10.1016/j.topol.2008.03.013