The water hole is the radio frequency range 1.42-1.66 GHz, bounded by the natural emission lines of neutral hydrogen (1.42 GHz) and hydroxyl (~1.66 GHz) — molecules that combine to form water. Long considered a relatively radio-quiet, “logical” band for interstellar communication, it has been the primary focus of most SETI narrowband surveys for decades. A 2026 ALMA archival survey by louisa-mason argues this concentration has left millimeter/submillimeter bands almost entirely unexplored for technosignatures (src-ras-alien-radio-channel-technosignature-2026-07).