Maximizing electromagnetic absorption in subwavelength structures is crucial for a wide range of engineering applications, from stealth technology to energy harvesting. Traditional approaches have focused on the design of sophisticated resonant structures, but less attention has been paid to the impact of the surrounding medium. In this work, we derive a universal and geometry-independent upper bound for the absorption cross section (ACS) of subwavelength structures immersed in arbitrary background media, quantitatively expressed as the inverse of the wavelength in the host medium. This theoretical framework further indicates that the bound diverges in the epsilon-near-zero (ENZ) limit, where the wave number of the background medium approaches zero. By leveraging waveguide-emulated plasmonics, we experimentally demonstrate an extreme ACS that surpasses the vacuum upper bound by more than tenfold via ENZ immersion. In ENZ media, the absorption can reach very large values and is accompanied by minimal local flow disturbance, suppressed scattering, and near-uniform energy distribution. ENZ immersion opens avenues for engineering applications, including ultraefficient sensors, advanced stealth coatings, and compact thermal management systems, significantly advancing the fields of microwave engineering, optics, and metamaterials.

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• Dielectric properties
• Permittivity
• Plasmonics
• Dielectrics