Alert Fire Seekers reader (and diver) David Jeffrey just pointed out to me that my description of divers using GPS to map an underwater site (in the Antikythera chapter, ‘Devil in the Deep Blue Sea’) is inaccurate, because GPS doesn’t work underwater. Apparently what you need is GPS to accurately locate the dive boat, and an acoustic system to locate the divers relative to the boat. Why I didn’t think of this, having recently experienced GPS not even working well under trees, I don’t know. But there you have it. As a novice (basic PADI cert) diver, describing a technical dive, I should have reviewed more carefully.