Between November and March, scalloped hammerhead sharks aggregate in large schools in the waters off Yonaguni — one of the most reliable places in Japan to encounter these animals in numbers. The dive sites where they congregate, particularly around the underwater seamounts on the island's western side, regularly produce encounters with schools of dozens of sharks circling in the blue water. It is not guaranteed — the sharks move with the current — but Yonaguni has one of the better track records in the Pacific for consistent sightings.
The dives are not for beginners. The sites are deep (20–30+ metres), currents are strong, and the sharks, while not aggressive toward divers, require calm and respectful behaviour. All the dive operators on the island offer hammerhead-specific dive packages during season. Staying for multiple days significantly improves your chances of a sighting. Out of shark season, the diving is still excellent: manta rays, sea turtles, and reef fish are present year-round.