Chuuk (Truk) Lagoon: Technical Diving the Hailstone Fleet

On 11 September 2025, oil was noticed leaking from the Rio de Janeiro Maru, one of Chuuk Lagoon's shallower and easier wrecks. Authorities confirmed it two days later. X-Ray Mag reports that heavy fuel oil reached the surface and threatened reefs and shorelines before the leak was contained. Operators kept clear of the area as a precaution, and no permanent closure was reported.

The leak was a reminder of what these wrecks are. The fleet sunk by the US Navy's Operation Hailstone on 17–18 February 1944 is not a static museum. It is more than 80 years of corroding steel, still holding mines, torpedoes, depth charges, occasional live ammunition and trapped fuel. A recent expedition described in DAN's Alert Diver found collapsed sections, crumbling decks and weakened steel beams.

For technical divers, that is both the appeal and the problem. Chuuk offers warm water, mild current, real depth and cargo holds that still read like a quartermaster's inventory. It also sits a long way from dependable hyperbaric care.

Killer features

A wreck field with no real equivalent

Dive The World counts more than 50 wreck sites. Other sources count more than 60 or 70.

The Past records that Operation Hailstone sank approximately 45–51 ships and destroyed or damaged about 250–275 aircraft. Most wrecks sit in three anchorages, generally 15–40 minutes by boat from shore, according to X-Ray Mag.

The San Francisco Maru's cargo

The "Million Dollar Wreck" sits upright and deep, and it is still loaded. Master Liveaboards lists Type 95 tanks, trucks, a steamroller, mines, detonators, bombs, shells, fuel drums, depth charges and Type 93 Long Lance torpedoes.

Aircraft in the Fujikawa Maru's holds

The Fujikawa Maru sits upright with her king posts reaching 9 m (30 ft). Her open holds contain Zero fighter remains and parts: engines, propellers, fuel cells and wheels. X-Ray Mag describes a mostly intact Zero fuselage in one hold and a coral-encrusted bow gun.

Interiors you can still read

Several wrecks allow entry to holds, engine rooms, bridges and passageways. The Kensho Maru's upper engine room has a two-storey ceiling reached through skylights. The Nippo Maru's bridge still holds its telegraph and steering binnacle.

The dives

Published depths vary between sources, so treat these as planning ranges and confirm on site. Depths below are from Dive The World, X-Ray Mag and Master Liveaboards.

Other sources place the San Francisco Maru anywhere between 50 m and 64 m, so plan for the deeper figure. Bluewater Travel cites one dive with 30 minutes of decompression. Another, to 52 m (171 ft), ran 65 minutes with 34 of them in deco.

A torpedo hit her forward hold at about 08:30 on 17 February 1944, and the explosion that followed nearly vaporised the bow. The Past records that approximately 945 troops and crew died, with one survivor.

Risks and how to manage them

Depth, decompression and narcosis

The risk: The technical sites run from 40 m to about 64 m. Master Liveaboards names depth, repetitive diving and decompression obligations as the DCI risks. One diver's account on the Flickr blog describes narcosis at 52 m (171 ft) as feeling like intoxication.

Manage it: Hold the certification for the depth and put helium in the mix for the deeper wrecks. Plan every dive, finish every stop, and stay conservative across a multi-day trip. Leave at least 24 hours between the last dive and the flight home.

Overhead environments and silt

The risk: Engine rooms are unlit, holds are narrow, and guided divers depend on the guide for the way out. Penetration of the I-169 is not recommended because of frequent silt-outs, and bubbles or poor buoyancy can ruin visibility inside a hold.

Manage it: Carry at least two torches and never penetrate alone. Both are written rules on at least one liveaboard. Guides lead only simple penetration within their own training, so technical teams plan and run their own. Hold trim, control your breathing and stay off the walls.

Deteriorating structure

The risk: Scuba Diving magazine reports that the shallower wrecks are deteriorating rapidly. DAN's account adds collapsed sections and interiors considered close to collapse.

Manage it: Treat any route you remember from an earlier trip as unverified. Ask the operator which interiors are currently off limits, and stay out of any space the briefing flags.

Ordnance and oil

The risk: The cargo includes mines, detonators, bombs, shells, depth charges and torpedoes. Bluewater Travel reports occasional live ammunition in the deep wrecks. Some wrecks still hold trapped oil, and Scuba Diving describes oil exposure as an immediate threat to divers.

Manage it: Touch nothing and move nothing. No source gives a clearance status for any wreck or names the sites holding dangerous oil. Follow the operator's avoidance calls, as happened after the Rio de Janeiro Maru leak.

Remote emergency care

The risk: Operator trip notes state that the Truk chamber is volunteer-manned and not guaranteed to be operational. DAN describes a privately operated chamber on Weno with basic equipment, where staff must be called in. Serious cases typically go to US Naval Base Guam, about 1,019 km (633 miles) away.

Manage it: Carry insurance that covers evacuation, chamber treatment and hospital care at the depths you plan to dive. If the chamber is unavailable, private air evacuation to Guam or Australia may be necessary.

Boats, moorings and separation

The risk: Currents are generally mild, but X-Ray Mag reports current and mooring-line position complicating an ascent on the Nippo Maru. Technical divers may ascend without a guide and must be back at the mooring in time for departure.

Manage it: Agree the ascent point before you descend and leave margin to reach the mooring. Carry two SMBs and two reels or spools with at least 15 m (49 ft) and 60 m (197 ft) of line.

Lessons from past incidents

Most documented diving incidents at Chuuk come from traveller reviews and forum posts, often without names, dates or causes.

Lost inside an engine room. A TripAdvisor review describes a guide finding two technical divers lost in an engine room in late 2015. The same source reports a fatality on the Hoyo Maru, where the diver was found in a small remote room after extensive searches. No name, date or cause is given. Run a continuous line and never penetrate alone.

DCI managed with surface oxygen. Reviews of the SS Thorfinn describe suspected decompression illness in October 2008 and September 2015. Both divers were given oxygen, and neither outcome is stated. What follows depends on a chamber that may not be running.

Surface support failures. An October 2014 review of the same operation describes a dive boat that broke down repeatedly and drifted off site, leaving three divers a long swim through rough water.

A death with no stated cause. A ScubaBoard thread reports the death of a Dirty Dozen Expeditions team member during a Chuuk trip. The cause was not stated, and her family asked that speculation be withheld.

Rules, permits and access

Removing artefacts from the wrecks is illegal, as Scuba Diving magazine reports. DAN describes the wrecks and aircraft as protected as a historic site, monuments and war graves. Sources differ on the formal legal designation, so confirm the current position with your operator. Human remains have been reported on the Yamagiri Maru. Dive every wreck here as a grave.

Permits and fees

• A Truk dive permit applies. Blue Lagoon's package includes it, and Odyssey lists it at US$50.
• Master Liveaboards collects taxes and permits onboard: US$195–250 depending on itinerary length.
• The Chuuk Airport departure tax is US$30, payable in cash.

Operator rules compared

OKTO Dive gives the general position: Advanced Open Water is generally required, with documented wreck training for certain penetrations. Confirm the rest in writing before you book.

Getting there

Chuuk is about two hours by air from Guam, on United Airlines. The alternative is the Honolulu Island Hopper via the Marshall Islands.

Planning notes

Gases

Nitrox fills of 30–32% are standard. Trimix is available at Blue Lagoon and on the Odyssey, and Blue Lagoon's 2024 flyer recommends trimix 16/45 as a CCR bottom mix.

Supply and prices change, so contact your dive operator before you book. Ask which gases, cylinders and sorb they can supply, and what each costs.

On open circuit, a typical plan might use the mixes below. Each keeps bottom PO2 at or under 1.3, equivalent narcotic depth (END) under 30 m with oxygen counted as narcotic, and gas density at or under the 5.2 g/L target.

All three are breathable from the surface. Leaner helium mixes are common, but check the density: 21/35 at 50 m and 17/50 at 65 m come in at 5.5–5.6 g/L, over the target though inside the 6.2 g/L maximum. Nitrox does not pass at 40 m: EAN27 gives an END of 40 m and 6.5 g/L. For CCR divers on a 1.3 setpoint, the 16/45 diluent works out at about 5.0 g/L at 50 m and 6.0 g/L at 64 m.

The examples assume decompression on EAN50 from 21 m and oxygen from 6 m. They are illustrations, not prescriptions. Run your own numbers in your own software.

Decompression approach

Gradient factors between 30/70 and 50/80 are common open-circuit settings for these profiles, and many CCR divers use 40/85. Divers planning on VPM-B commonly add +2 or +3 conservatism. Consider a lower GF-high for repetitive, multi-day diving. Master Liveaboards suggests a 5 mm suit for long stops, even in 28°C water.

Operators

Scuba Diving magazine lists a seven-day Odyssey charter at US$3,195 before tax and permit.

Staying ashore: Blue Lagoon Resort

Blue Lagoon Dive, at Blue Lagoon Resort on the southern point of Moen Island, is the shore-based alternative to a liveaboard. The deeper sites are 15–20 minutes away by boat. The shop has CCR storage, a booster pump, an oxygen generation plant, helium, sorb, deco tanks and manifolded twins.

The flyer's group package was US$3,895 for 12 diving days, 20 dives and 13 nights twin share, with airport transfers and the dive permit included. Oxygen, helium, sorb and CCR cylinder hire were extra. Small boats and a resort base suit CCR teams and long decompression dives.

Insurance and evacuation

Buy cover that names technical diving at your planned depths, plus evacuation, chamber and hospital costs. Build the 24-hour no-fly window into your flights.

Bottom line

Chuuk suits two kinds of technical diver. The first holds Advanced Nitrox and decompression certification and wants a week or more of wrecks in the 30–45 m range, with penetration as an option. The second is the normoxic trimix or CCR diver aiming at the San Francisco Maru, the Aikoku Maru and the deeper holds of the Hoki and Nippo.

Get the training before you travel. Operators expect proof of qualification for depth and for penetration, and at least one liveaboard runs no courses in Chuuk. Add formal wreck penetration training if you plan to go inside.

Then choose your base: Blue Lagoon for small boats and a flexible schedule, or a liveaboard moored directly over the deep wrecks.

It suits least the diver who counts on a chamber being close. Here it may not be.

Chuuk is number one in our Top 25 technical dive locations.

Safety disclaimer

Information, not instruction. Nothing here replaces training with a qualified instructor. Depths, conditions, prices and rules come from published sources and change over time. Gas mixes and decompression settings shown are illustrative only. Plan every dive with your own software, within your own training and experience, and confirm current conditions and requirements with your operator.