The Japanese Type A kō-hyōteki miniature (mini or midget) submarines were built from 1940 to 1943 in Japan. These mini submarines had an overall length of 23.9 meters, a beam of 1.9 meters, a displacement of 46 tons submerged, and a test depth of 30 meters. They were equipped with two 450 millimeter torpedoes loaded into two tubes on the bow and one 140 kilogram scuttling charge. 

This particular mini submarine was identified as No. 20, one of five that took part in the Japanese attack on Pearl Harbor on December 7, 1941. On the morning of the attack on Pearl Harbor, USS Ward, a Wickes-class destroyer, sighted No. 20’s periscope as it was following the American cargo ship Antares. Ward attacked No. 20 with its 4-inch guns and landed a hit at the base of the conning tower, sinking the submarine. This was the first American-caused casualty of the Pacific Theater during World War II. 

The Hawaiʻi Undersea Research Laboratory rediscovered No. 20 on August 28, 2002. On December 7, 2016, NOAA Ocean Exploration and NOAA’s Office of National Marine Sanctuaries visited and explored No. 20 on the 75th anniversary of the attack on Pearl Harbor during a shakedown expedition aboard NOAA Ship Okeanos Explorer. Remotely operated vehicle (ROV) Deep Discoverer — equipped with an Insight Pacific Zeus Plus camera capable of collecting high-definition footage — was used to document the site, which is at a depth of approximately 400 meters. 

The footage used for the two photogrammetric models of No. 20 was from timestamps 16:40 to 17:55. This footage was processed into still images using Photoshop and was not color corrected. 1,496 images were used for the two models of the site. The footage collected during exploration of No. 20 was not intended for photogrammetry. While aligning the model, there was some distortion at the stern and misalignment on the port and starboard sides. Due to this, individual port and starboard models were made.

The full annotated video of this dive can be viewed on SeaTube.

Site Name: Japanese Miniature Submarine (NO-20)

Site Type: UCH

UCH Vessel Date Built: 1940

UCH Vessel Date Sank: December 7, 1941

Hull Material: Steel

Official Number: No. 20

Expedition Number: EX1608

Expedition Name: Shakedown, Calibration, and Testing in the Main Hawaiian Islands

ROV Dive Number: 05

ROV Dive Date: December 7, 2016

Location: South of Oahu

Depth: 400 meters

Length: 23.9 meters

Width: 1.8 meters

ROV Used: Deep Discoverer

Camera Information: Insight Zeus Plus HD, 3-CCD color camera with 2/3-inch 2,200,000 pixel 1080i IT CCDs

Video or Stills: Video

Number of Images Used/Format: 1,496

Image Alignment Percentage: 85%

Number of Tie Points: 148,175

Link to Raw Images:

Time to Complete: 4 hours

Orthomosaic Views Available: No

Images Available: Yes

Animations Available: Yes

Available File Exports/Location/POC:

Link to NOAA Ocean Exploration Project Page:

Software: Agisoft Metashape Standard Version 2.0.1.

Developer: Raymond Phipps, NOAA Ocean Exploration explorer-in-training, July 10, 2023.

Credit: Model courtesy of NOAA Ocean Exploration, Shakedown, Calibration, and Testing in the Main Hawaiian Islands.