Urea is a prototypical hydrogen-bonded small molecule that has stimulated considerable fundamental research into its structure, elastic properties and polymorphism. Amongst the high-pressure phases, the structure of urea II is unknown. Our DFT calculations on urea III reveal an uncommon elastic property, which is negative linear compressibility (i.e., one direction in the crystal expands when the crystal as a whole is compressed). This has recently been observed experimentally. We propose to use new STFC funded apparatus to form urea II and urea III and then recover them to room pressure under liquid nitrogen. This allows us to obtain the highest-resolution neutron powder diffraction data, which will enable structure solution of urea II and high-precision thermal expansion measurements of both polymorphs. We will correlate the thermal expansion with H-bonding and high-P compressibility.