Mesopores become micropores under pressure (10-50 MPa), which is applied to dahlia-shaped arrays of single-wall carbon nanohorns (SWNHs, see Figure for a TEM image) in liquid media, such as ethanol or glycerol. While the morphology of the arrays does not substantially change, the apparent volume shrinks by a factor of 20 and micropores develop. An improved procedure may even provide a route to supercritical gas absorbers.