You dive under a ceiling of drift ice in water below freezing, looking up at blue-white cathedral light through the floes…You dive under a ceiling of drift ice in water below freezing, looking up at blue-white cathedral light through the floes, hunting for clione, 'sea angels', translucent 2cm swimming snails that have become a Japanese icon. Above water, Steller's sea eagles and Blakiston's fish owls, and orcas in the strait in summer. The Sea of Okhotsk drift ice reaches Hokkaido each winter, the lowest-latitude sea ice in the northern hemisphere. Shiretoko became a UNESCO World Heritage site in 2005 for exactly this, an ecosystem driven by seasonal ice. Ice cover has been measurably declining, and operators say the season is shortening. Anchorage: Rausu or Utoro harbour. The Nemuro Strait fills with drift ice Feb-Mar, navigation is seasonal and ice-dependent. Hazards: Sub-zero water under a moving ice ceiling, drysuit, ice-rated regulators, surface tether and a specialist operator are all mandatory. Regulator freeze-up is the real risk. Meer