"...the British were bent on destroying before they left the district [about 24 August 1814]
"A squad of soldiers was on the line of march toward the foundry while the public buildings were burning. They had reached the bridge thrown over the little creek which separates Washington from Georgetown, when they were called to a sudden halt by a tremendous explosion. Soon after a courier came dashing up at full speed, warning them not to cross the bridge; that the explosion just heard was from Greenleaf's Point, a fort on the opposite side of the Potomac from Washington, recently vacated by the Americans. One hundred and fifty British soldiers had been blown up, their mangled remains flying in every direction.
"The fact was, several barrels of powder had been thrown into a dry well by the garrison before leaving, to keep it from falling into the hands of the enemy. The detachment sent over to destroy the fort were dying of thirst and one of the men threw a torch into the well to ascertain if it contained any water, hence the catastrophe. [Source]