It's called Sonar.
Sonar
sonar
They used to train dolphins with cameras.
detector diodes in RADAR sets
during World War 2
underwater submarine
The Piranah Water 3000
It means:Pipeline Underwater Transport of Oil
Yellow -turning pink in the presence of mustard gas.
10 mph underwater 20 mph surface
troop transports, hospital ships, mining of harbours, underwater demolition
They where U-boats or Underwater Boats. These ships sunks most likely the most ships in the entire war.