Highly ultramicroporous single-walled carbon nanohorns (see Figure), porosity-designed by HNO3 treatment, show dramatically enhanced storage capacity of supercritical methane, which can be adsorbed into internal and interstitial spaces of the single-wall carbon nanohorns. This enhanced storage capacity of methane may result from the developed ultramicroporosity, which can promote the adsorptivity of methane.