American troops using a newly-developed acoustic locator, mounted on a wheeled platform. The large horns amplified distant sounds, monitored through headphones worn by a crew member, who could direct the platform to move and pinpoint distant enemy aircraft. Development of passive acoustic location accelerated during World War I, later surpassed by the development of radar in the 1940s. #

An Austrian armored train in Galicia, ca, 1915. Adding armor to trains dates back to the American Civil War, used as a way to safely move weapons and personnel through hostile territory. #

The interior of an armored train car, Chaplino, Dnipropetrovs'ka oblast, Ukraine, in the spring of 1918. At least nine heavy machine guns are visible, as well as many ammunition cases. #

Allied advance on Bapaume, France, ca. 1917. Two tanks are moving towards the left, followed by troops. In the foreground some soldiers are sitting and standing at the roadside. One of them appears to be having a drink. Beside the men is what appears to be a rough wooden cross with an Australian or New Zealand service hat on it. In the background other troops are advancing, moving field guns and mortars. #

Soldier on a U.S. Harley-Davidson motorcycle, ca. 1918. During the last years of the war, the United States deployed more than 20,000 Indian and Harley-Davidson motorcycles overseas. #

British Medium Mark A Whippet tanks advance past the body of a dead soldier, moving to an attack along a road near Achiet-le-Petit, France, on August 22, 1918. The Whippets were faster and lighter than previously deployed British heavy tanks. #

A German soldier rubs down massive shells for the 38 cm SK L/45, or "Langer Max" rapid firing railroad gun, ca. 1918. The Langer Max was originally designed as a battleship weapon, later mounted to armored rail cars, one of many types of railroad artillery used by both sides during the war. The Langer Max could fire a 750 kg (1,650 lb) high explosive projectile up to 34,200 m (37,400 yd). #

German infantrymen from Infanterie-Regiment Vogel von Falkenstein Nr.56 adopt a fighting pose in a communication trench somewhere on the the Western Front. Both soldiers are wearing gas masks and Stahlhelm helmets, with brow plate attachments called stirnpanzers. The stirnpanzer was a heavy steel plate used for additional protection for snipers and raiding parties in the trenches, where popping your head above ground for a look could be lethal move. #

Turkish troops use a heliograph at Huj, near aza City, in 1917. A heliograph is a wireless solar telegraph that signals by flashes of sunlight usually using Morse code, reflected by a mirror. #

An experimental Red Cross vehicle designed to protect the wounded while gathering them from trenches during World War I, ca. 1915. The narrow wheels and low clearance would likely make this design ineffective in the chaotic and muddy front line landscape. #

U.S. soldiers in trench putting on gas masks. Behind them, a signal rocket appears to be in mid-launch. When gas attacks were detected, alarms used included gongs and signal rockets. #

A disused German trench-digging machine, January 8, 1918. The vast majority of the thousands of miles of trenches were dug by hand, but some had mechanical assistance. #

Western front, loading a German A7V tank onto a railroad flat car. Fewer than a hundred A7Vs were ever produced, the only tanks manufactured by Germany that they used in the war. German troops did manage to capture and make use of a number of allied tanks, however. #

Americans setting up a French 37mm gun known as a "one-pounder" on the parapet of a second-line trench at Dieffmattch, Alsace, France, where their command, the 126th Infantry, was located, on June 26, 1918. #

On the site where a steel bridge was destroyed, a wooden temporary bridge has been built in place. Note that an English tank which fell in the river when the former bridge was demolished now serves as part of the foundation for the new bridge over the Scheldt at Masnieres. #

An unidentified member of the 69th Australian Squadron, later designated No. 3 Australian Flying Corps, fixes incendiary bombs to an R.E.8 aircraft at the AFC airfield north west of Arras. The entire squadron was operating from Savy (near Arras) on October 22, 1917, having arrived there on September 9, after crossing the channel from the UK. #

Seven or eight machine-gun crews are ready to set out on a sortie in France, ca. 1918. Each crew consists of two men, the driver on a motorbike and the gunner sitting in an armored sidecar. #

German troops load gas projectors. Attempting to exploit a loophole in international laws against the uses of gas in warfare, some German officials noted that only gas projectiles appeared to be specifically banned, and that no prohibition could be found against simply releasing deadly chemical weapons and allowing th wind to carry it to the enemy. #

French lookouts posted in a barbed-wire-covered trench. The use of barbed wire in warfare was recent, having only been used for the first time in limited form during the Spanish-American War. All sides in World War I used extensive networks of barbed wire entanglements to prevent ground troops from moving forward. The effectiveness of the wire drove the development of technologies like the tank, and wire-cutting explosive shells set to detonate the instant they made contact with a wire. #

The original caption reads: "The Italian collapse in Venezia. The heedless flight of the Italians to the Tagliamento. Captured heavy and gigantic cannon in a village behind Udine. November 1917". Pictured is an Obice da 305/17, a huge Italian howitzer, one of fewer than 50 produced during the war. #