Until the middle of the 20th century there was no method to measure currents below the surface of the open ocean. The problem was solved in 1955 when a British oceanographer, John Swallow, developed what became known as the neutrally buoyant (or Swallow) float.
The Swallow float would stay at a fixed depth and move with the currents at that depth. The path taken by the float, and hence the current speed and direction, could be measured by scientists on a nearby ship listening to sounds ('pings') emitted at regular intervals from the float.
Information from Swallow floats soon revealed that the deep oceans were not the tranquil regions predicted from theory. Instead they were filled with energetic currents that vary on scales of weeks and hundreds of kilometres, much as the atmosphere is filled with weather systems that change the speed and direction of the winds.
By the 1970s much bigger floats were being tracked using lower frequency sound and fixed listening stations. These SOFAR floats were used to produce maps of the underwater 'weather' but only in areas where there was a network of tracking stations, not throughout the world.
In the 1990s the World Ocean Circulation Experiment (WOCE), part of the World Climate Research Programme, needed to measure subsurface currents globally. The measurments would complement the Experiment's ship-based survey and data from radar altimeter satellites.
To meet WOCE's neeeds, Doug Webb and Russ Davis in the USA developed the Lagrangian Ocean Circulation Explorer (ALACE) - a neutrally buoyant float that came to the surface every 2 weeks to have its position determined by satellite. Later in WOCE the ALACEs were modified to measure temperature and then salinity profiles each time they surfaced. It was this technology that allowed Argo to be planned.
In 1999 a global array of 3000 profiling floats was proposed to the Oceanobs'99 conference, and in 2001 the first Argo floats were deployed. Inspired by the Greek myth of Jason and the Argonauts, the floats were given the name Argo to stress the way they would work with the new Jason satellite altimeter, launched in December 2001.
The floats provide the vertical structure of temperature, salinity and current velocities that is needed for successful interpretation the sea surface height measurements from the altimeter. In turn satellite altimetry data give frequent and closely spaced measurements of sea surface height that are beyond the capability of an in situ observing system. Together they make it possible to understand and even predict the 'under-water weather systems' first revealed by the Swallow floats.
By November 2007 a global fleet of 3000 Argo floats were operating. Jason II was launched in June 2008, and is now one of several altimeters that together with the Argo float measurements give us a complete description of water movement in the upper ocean.