  Cryogenic transmission electron microscopy has revolutionized structural
biology and materials science, but achieving temperatures below the boiling
point of liquid nitrogen remains a long-standing aspiration. We introduce an
ultra-cold liquid helium transmission electron microscope specimen holder,
featuring continuous cryogen flow and vibration decoupling. This instrument is
compatible with modern aberration-corrected microscopes and achieves sub-25 K
base temperature, ${\pm}$2 mK thermal stability over many hours, and atomic
resolution--setting the stage for a new era of cryogenic electron microscopy.
