A hi-tech "worm" is probing a subterranean pipeline used to discharge radioactive effluent from Dounreay between 1957 and 1992.
The £100,000 pipe crawler has sent back video and radiation readings during its five-day journey some 45 metres underground.
The data will be analysed by a project team investigating how to leave the disused system in a safe condition as part of the site clean-up.
A bundle of four cast iron pipes, each 23cm in diameter, was laid in the 1950s to discharge effluent from the fast reactor experiment.
It was connected on the surface at one end to two disused tanks where effluent drained from the reactors, chemical plants and waste facilities.
The pipeline bundle descends towards the sea along a sloping adit to a tunnel excavated 25 metres beneath the seabed.
At the end of the tunnel, some 600 metres offshore, the pipeline terminated in a diffuser attached to vertical risers that exited on the seabed 20 metres beneath the waves.
The pipes were encased in a thick concrete sleeve that runs along one side of the tunnel. When construction finished in 1957, the tunnel was abandoned and allowed to flood.
"The tunnel system has been flooded for more than half a century and the pipeline itself was taken out of routine use in 1992, so we don't know what condition the pipeline is in or if it safe to leave in place," explained Martin Howse, project manager with Dounreay Site Restoration Ltd.
"The remotely-operated vehicle will allow us to inspect the inside of the pipes. We are looking for signs of structural degradation, trapped debris and radioactive contamination.
"The findings will allow us to reach some judgements about the best course of action. It is our intention to present a proposal to the Scottish Environment Protection Agency by December."
The performance of the subsea discharge system started to deteriorate in the late 1970s. It was taken out of routine use in 1992 after a new plastic pipeline was installed in the tunnel and connected to a modern diffuser on the seabed.
Two companies with specialist remote inspection skills honed in the North Sea are carrying out the survey - MSIS and Hydropulsion.
The head of the "worm" is 2.5 metres long and 15cm in diameter. Its flexible tracked chassis can turn through bends in the pipes. An umbilical "tail" allows the pipeline crawler to be controlled remotely from the surface, where the project team can view "live" video and radiation readings.
