A medium sized freight vessel, of which's sinking remains a bit vague. She must have had a direct hit in the aft holds, or had raging fires which reached the ammunition stored in her after holds. There is not much left of the superstructure and aft holds, but the forward holds still hold a lot of interesting stuff to look at.

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284 Depth charges where removed from the wreck to make it safer for divers. 

Ship History (Source www.pacificwrecks.com) 
Sankisan Maru was owned by Nippon Yusen Kaisha (NYK) and used as a passenger and cargo vessel with a 6,000 ton capacity. Prewar, used to transport rice from Indo-China (Vietnam), Korea and China. During one trip from Japan, transported six locomotives and 54,000 yards of canvas to Phonom Penh.

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Wartime History
Requisitioned by the Japanese Army as a cargo vessel (AK) to transport military cargo armed with machine guns for anti-aircraft defense. On June 4, 1942 departed Tokyo Bay bound for Marifu as part of a convoy with Eiyo Maru, Hokko Maru and Taian Maru escorted by destroyer Hatakaze.

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On September 5, 1943 departs Moji bound for Takao as part of convoy no. 194 with Juyo Maru, Kyokuto Maru, Tsushima Maru, Wazan Maru, Yulin Maru, Zuiyo Maru plus another unidentified merchant ship, escorted by destroyer Sanae.

On October 26, 1943, departs Yokosuka as part of convoy No. 3026 with Choan Maru No. 2, Soyo Maru, Awa Maru escorted by Fukue. On October 28, 1943 Sankisan Maru suffers a mechanical break down and is escorted by Fukue to Chi Chi Jima arriving two days later at 1:30pm and repaired.

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During 1943 operated off Saipan. On November 18, 1943 departs Truk Lagoon at 10:00am as part of a convoy with Hinode Maru escorted by minesweeper W-28. On December 2, 1943 at 6:00am departs Chi Chi Jima as part of a convoy with Hinode Maru escorted by Patrol Boat No. 46 and minesweeper W-28 bound for Yokosuka.

During January 23-31, 1944 at Yokosuka. On January 31, 1944 at 7:00am departs Yokosuka bound for Truk Lagoon as part of a convoy with Zuikai Maru, Ryuko Maru and and Taiho Maru escorted by subchasers CH-29, CH-64 and auxiliary subchaser Takunan Maru No. 6 and and auxiliary minesweeper Keinan Maru. En route during February 1944 subchaser CH-32 joins the convoy to provide additional escort. On February 12, 1944 Sankisan Maru arrived at Truk Lagoon.

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Sinking History
On February 17, 1944 during "Operation Hailstone" anchored to the west of Uman Island in Truk Lagoon loaded with ordnance and supplies. During USS Bunker Hill Strike 3D, four SB2C Helldivers from VB-17 attacked Amagisan Maru then attacked this vessel, hitting it in the forward deck with a 1,000 lb bomb and strafed by two escorting F6F Hellcats from VF-18 firing 400 rounds at an "AK south of Fefan Island".

The next day on February 18, 1944 during "Operation Hailstone" this vessel was again targeted by USS Bunker Hill Strike 3A by five SB2C Helldivers attacked "an AK at anchor about a half mile west of Uman Island " and scored a hit to the aft with a bomb or torpedo that caused an explosion that damaged the midship and caused the vessel to sink. The stern section was blown off and sank into deeper water. Possibly, the hit or fire caused ordnance stored in the aft hold to explode. The explosion caused a crater in the sea bed. The precise history of the sinking is not clearly documented in Japanese records.