  We report on the extension of the spectral sensitivity of superconducting
nanowire single-photon detectors to a wavelength of 29 $\mu$m. This represents
the first demonstration of a time correlated single-photon counting detector at
these long infrared wavelengths. We achieve saturated internal detection
efficiency from 10 to 29 $\mu$m, whilst maintaining dark count rates below 0.1
counts per second. Extension of superconducting nanowire single-photon
detectors to this spectral range provides low noise and high timing resolution
photon counting detection, effectively providing a new class of single-photon
sensitive detector for these wavelengths. These detectors are important for
applications such as exoplanet spectroscopy, infrared astrophysics, physical
chemistry, remote sensing and direct dark-matter detection.
