The role of bathymetry and submarine warfare in the Spratly Islands dispute has been largely ignored. To better understand this aspect of the dispute, it is necessary to dispel the myth of the Spratlys as an area of shallow sea, extremely dangerous for navigation and to be avoided at all costs. This perception, common before the 1920s, should have dissipated following secret missions by the colonial powers.
The British Admiralty organized several covert hydrographic missions between 1925 and 1938, meticulously mapping the area then known as the “Dangerous Ground.” In 1934 it discovered a safe sea-lane, crossing this area from north to south. The Japanese Imperial Navy also explored this maritime space from 1936 to 1938 and drew secret charts, notably of the main island, Itu Aba. The U.S Navy, based in Cavite, Philippines, also explored the Dangerous Ground, drawing its own secret bathymetric charts between 1935 and 1937 and discovering another sea-lane, crossing the area from east to west.
The Japanese commandant Unosuke Kokura, summarized perfectly, on May 1939, the gap between the civilian and military perceptions of the Spratlys:
A remarkable fact is that the whole of the Spratly islands can be considered as a kind of fortified area, because it is known as a dangerous area on all the maps of the world… But because of a laborious work of our Imperial Navy, this area is not a dangerous area at all for us. Our warships and commercial ships can sail freely through these groups of islands and take shelter behind the reefs.
The secret hydrographic research during the 1930s allowed naval authorities from different countries to understand this vast maritime territory as an archipelago crossed by secret sea-lanes. The Spratly Islands, long perceived as an area to avoid, were reinterpreted as strategic territory from which a maritime power could control the sea-lanes of the South China Sea. During the Second World War, U.S. submarines based in Australia patrolled the South China Sea and regularly crossed the Dangerous Ground using these internal sea-lanes. They also pursued scientific research around the Spratlys, notably to study layers of high density salt in the water which allowed the American submarines to escape detection by Japanese sonar.