The Allegheny Arsenal was established in 1814 during the War of 1812 and operated as a military facility until 1926. Located on approximately 38 acres in what is now the Lawrenceville neighborhood of Pittsburgh, the arsenal manufactured and stored ammunition, weapons, and other military equipment through the life of the facility. During the Civil War, the arsenal served as a critical supply center for the Union Army. On September 17, 1862, a devastating explosion at the facility claimed the lives of 78 workers, primarily young women assembling cartridges.
This tragedy represents one of the worst civilian disasters of the Civil War era. Following the Civil War, the Arsenal began accepting cannonballs for storage that were eventually stacked outside along the parade grounds for approximately 60 years.
In 1902, the Arsenal officially closed and started selling property off to the southeast of Butler Street for a Marine Hospital and city park to commemorate the accident at the arsenal. In the years to follow, the Arsenal was renamed as the Pittsburgh Storage and Supply Depot and reduced to a 19.75-acre portion northwest of Butler Street. In 1917, a large construction project began at the Storage Depot to build new warehouses. The Allegheny Arsenal Handbook by Tom Powers and James Wudarczyk states that the stacked cannonballs were buried in trenches in 1917 during the construction of the new warehouses. The cannonballs were forgotten about for ~50 years until 1972 when a cannonball burial pit was encountered. Since 1972, 4 additional
cannonball burial pits were reported during 2017 and 2020 redevelopment activities (construction of the Arsenal 201 apartment buildings).

Even if they are old, munitions can be dangerous. If you encounter an object that is unfamiliar to you, protect yourself and your family by learning the simple 3Rs of explosives safety:

Recognize - when you may have encountered a munition and that munitions are dangerous

Retreat - do not approach, touch, move or disturb it, but carefully leave the area

Report - call 911 and advise the police of what you saw and where you saw it