Abstract At its peak, the Eastern Front encompassed the entire frontier between the Russian Empire and Romania on the Allied side and the Austro-Hungarian Empire, Bulgaria, the (Turkish) Ottoman Empire and the German Empire on the other: a distance of c. 1500–1700 km. Mobile warfare alternated with periods of static trench and siege warfare, when ‘mines’ (tunnels charged with explosives detonated to breach the overhead fortifications) and ‘dugouts’ (to protect troops from artillery or aerial bombardment) were constructed in at least 50 localities between 1915 and 1917, from Mitau in present-day Latvia to Rarancze in Moldova. Terrain ranged from plateaux floored by Cenozoic sediments in the north to mountains with more complex, older, stronger rocks in the south. Despite lessons learnt from the Russo-Japanese war of 1904–05, both sides began the conflict unprepared for tunnelling warfare. However, German and Austro-Hungarian forces soon developed units of military geologists whose duties included guidance of tunnelling projects. Eight teams of German military geologists and four of Austro-Hungarian, in total over 60 men who can be named, are known to have served on the Front, as did newly formed and equipped specialist engineer mining battalions from Prussia, Bavaria and Austria-Hungary.