Half of it is tail, which it uses as a whip to stun fish. It spends the day deep and comes up to cleaning stations.
To about 4.6 m, of which nearly half is the upper lobe of the tail. The eye is enormous and set into a socket that extends onto the top of the head, giving it upward-looking vision, and there is a deep groove running back from above each eye. The common thresher lacks that groove and has a much smaller eye.
The tail is a weapon. It swims into a school of fish and brings the upper lobe over its own head like a whip, stunning several at once, and the strike has been filmed. It spends the day deep — several hundred metres — and comes up at night and to cleaning stations, which is where it is seen. At the Galapagos and Cocos that is an early-morning dive on a seamount at the limit of recreational depth, and the sighting is a matter of luck. It is listed as vulnerable and it survives capture poorly.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.