The site here is the Mitchell Monument erected in memory of five fatalities on this spot due to a Japanese fire bomb.  

Fire balloon

A fire balloon (風船爆弾, fūsen bakudan, lit. "balloon bomb"), or Fu-Go, was a weapon launched by Japan during World War II. A hydrogen balloon with a load varying from a 12-kilogram (26 lb) incendiary to one 15 kg (33 lb) antipersonnel bomb and four 5 kg (11 lb) incendiary devices attached, they were designed as a cheap weapon intended to make use of the jet stream over the Pacific Ocean and wreak havoc on Canadian and American cities, forests, and farmland.

• http://en.wikipedia.org/wiki/Fire balloon