Ai Weiwei on Friday unveiled his commission for the Museum of Contemporary Art Australia in Sydney: 500 cast-iron combat helmets arranged in ten orderly lines on the city’s busiest green space, Tallawoladah Lawn.

The 131-foot-long installation, titled Mushrooms, recalls various 20th-century combat helmets, complete with inner padding. The installation is arranged on an elevated platform split by a pedestrian walkway, making the work difficult for visitors to the famous harbor to miss. According to a statement from the artist, its title—like the curved lip of the helmet—alludes to “growth in darkness, nuclear mushroom clouds, and in Chinese, the term also suggests endless delays.”