Operation Hailstone lasted two days and left the Japanese fleet on the bottom of a lagoon in Micronesia. Eighty years later it is the densest concentration of accessible shipwrecks anywhere.

Why it is unlike other wreck diving

Most wrecks you can dive have been picked over, salvaged, swept by storms or deliberately sunk clean. Chuuk was too remote and too deep to strip. So the holds still contain what they were carrying: trucks, aircraft engines, shells stacked as they were loaded, gas masks, bottles, a bicycle. Nothing was arranged for a visitor, which is precisely why it reads as real.

The part divers do not talk about enough

These are graves. Several hundred people died in those hulls, and remains are still present in some of them. Chuuk has rules about penetration and about photography for that reason, and they deserve more respect than they usually get. A wreck being interesting does not make it yours.

What it demands of you

• Depth. Many of the best-known hulls sit between thirty and fifty metres. That is decompression diving, not a holiday dive with a rented BCD.
• Overhead training. Penetration is a distinct skill with a distinct failure mode: silt-out, where you cannot see your own hand and the way you came in is no longer obvious.
• Honesty. The commonest way this goes wrong is a diver saying yes to a dive they can nearly do.

The film below is the first part of Jonathan Bird's two-parter on Chuuk, which shows the interiors better than any photograph I could describe.