The European spacecraft Mars Express arrived in Mars orbit in december 2003 to study the planet. It included a giant radar called MARSIS which was designed to look below Mars' surface using long wavelength radio waves. In order to produce the long wavelengths you need a very large aerial, in this case 40m long. This was deployed explosively which the engineers were very worried would destroy the rest of the spacecraft. In the middle of 2005 they finally deployed the aerial which was sucessful.
However from features on Mars there must have been ten times more water than this so scientists are still looking for the remainder.