The anti-tank ditches near Ukraine's occupied southeastern town of Polohy stretch for 30 km (19 miles). Behind are rows of concrete "dragon's teeth" barricades. Further back are defensive trenches where Russia's troops will be positioned.
The defences visible in satellite imagery taken by Capella Space are part of a vast network of Russian fortifications sweeping down from western Russia through eastern Ukraine and on to Crimea built in readiness for a major Ukrainian attack.
Graphic: How Russia has heavily fortified swathes of Ukraine
Thousands of Ukrainian troops have been training in the West to use different military assets on the battlefield in a combined way ahead of a counteroffensive Ukrainian officials say will come when its forces are ready.
Reuters has reviewed satellite images of thousands of defensive positions inside both Russia and along Ukrainian front lines that show it is most heavily defended in the southern Zaporizhzhia region and the gateway to the Crimean Peninsula.
Six military experts said the defences, mostly built in the wake of Ukraine's rapid autumn advances, could make it harder for Ukraine this time and that progress would hinge on its ability to carry out complex, combined operations effectively.
"It's not the numbers for the Ukrainians. It's can they do this kind of warfare, combined arms operations?" said Neil Melvin, an analyst at the Royal United Services Institute (RUSI). "The Russians have shown they can't do it and they've gone back to their old Soviet method of attrition."
A Ukrainian counteroffensive could change the dynamics of a war that has slowed into a bloody battle of attrition and military experts say the length of the front could stretch Russia's defences.
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