By Steven McKenzie
Highlands and Islands reporter, BBC Scotland news website
Aerial photographs revealed the system of trenches. Pictures: RCAHMS
Remains of trenches dug by troops stationed in the Highlands have survived 90 years after World War I, experts said.
Their existence is largely unknown and the Royal Commission on Ancient and Historic Monuments (RCAHMS) only documented them about two years ago.
Aerial photographs revealed the system in a field in Ross-shire, prompting a closer inspection by the commission.
Allan Kilpatrick, of RCAHMS, said they may have been dug for training.
He said two separate sets of trenches appear to have been dug - one representing British lines and the other German - with 400 yards of "no man's land" between them.
I have seen some aerial photographs of some of the trenches and, while they are something you would expect to find on the Western Front, to see them in the UK is quite spooky
Historian, Imperial War Museum
Depths vary from 2ft to 5ft and some stretches have become overgrown with bracken.
A more spectacular system could be seen near Cromarty, but they were ploughed over during the late 1970s, or early 1980s.
Mr Kilpatrick said: "Practice trenches were excavated wherever there were soldiers, usually this was on military land."
He said little was known of those found in Ross-shire but they could be connected to the Royal Navy's massive presence in the port of Invergordon.
Mr Kilpatrick said: "Half the Grand Fleet was stationed in Invergordon and the other in Scapa Flow.
"It's a guess, but the trenches may have been dug by Royal Marines."
Nick Hewitt, a historian at the Imperial War Museum in London, said while remains of trenches were relatively common around the UK the wider general public were not aware of them.
He said: "I have seen some aerial photographs of some of the trenches and, while they are something you would expect to find on the Western Front, to see them in the UK is quite spooky."
He added: "Given the scale of the armed forces at the time of the First World War, there was a lot of trenches being dug around the country for training.
"There are trenches in Hampshire, the Peak District and the Pennines.
"One of my colleagues has told me of one near Blackpool which was a demonstration trench and people could come along and pay a couple of bob to watch the soldiers training."
Mr Hewitt said the training trenches were far from "glorified ditches".
A more spectacular system was ploughed over.
He said: "They were deep, had fire steps, bunkers, they zig-zagged and had communication trenches. They were complicated and before going to France the best thing the soldiers could do was practice digging them.
Invergordon was important to the navy before, during and after World War I and provided access to fuel and dockyard repairs.
Warships were anchored in the Cromarty Firth.
In 1915, HMS Natal blew up while anchored in the firth and 300 onboard were killed.
Trench warfare dominated World War I.
After the Battle of the Marne in September 1914, the German army was forced to retreat after failing to bring about the surrender of France.
Rather than give up territory which they already held, the Germans dug in to protect themselves.
The Allies could not break through so followed their example.
Trench lines soon spread from the North Sea to Switzerland.