The Mystery of Celtic Wood refers to the apparent disappearance without trace of 71 men of the 10th Battalion of the 1st Australian Division during a diversionary attack on German positions in Celtic Wood, near Passchendaele in West Flanders, during the Battle of Poelcappelle in the First World War. Official Army reports of the action state that investigations have failed to account for the fate of 37 men of the 10th Battalion. German records contain no mention of the attack, which led to speculation that the men were massacred and buried in a mass grave. Visitors to the site today are still given this explanation by guides. Rumours persist that the men had simply walked into a mist and disappeared. Some investigators attribute the lack of record of the missing to confusion, mis-reporting and clerical error.

The 10th Battalion was an infantry battalion of the Australian Army that served as part of the all-volunteer Australian Imperial Force during World War I. Among the first units raised in Australia during the war, the battalion was recruited from South Australia in August 1914 and formed part of the 3rd Brigade, 1st Division. After basic training, the battalion embarked for Egypt where further training was undertaken until the battalion was committed to the Gallipoli campaign. During the landing at Anzac Cove, it came ashore as part of the initial covering force. Members of the 10th Battalion penetrated the furthest inland of any Australian troops during the initial fighting, before the Allied advance inland was checked. After this, the battalion helped defend the beachhead against a heavy counter-attack in May, before joining the failed August Offensive. Casualties were heavy throughout the campaign and in November 1915, the surviving members were withdrawn from the peninsula. In early 1916, the battalion was reorganised in Egypt at which time it provided a cadre staff to the newly formed 50th Battalion. It was transferred to the Western Front in March 1916, and for the next two-and-a-half years took part in trench warfare in France and Belgium until the Armistice in 1918. The last detachment of men from the 10th Battalion returned to Australia in September 1919.