Before the 20th century, memorials were erected mostly to celebrate victory. Now, they are more often used to commemorate suffering — it is hard to imagine anyone building a triumphal arch these days — and have become a requisite stage in the collective grieving process, a means of dealing with cultural trauma, and perhaps a way of starting to move on. It’s almost as if, by consigning a memory to stone, we can begin to forget.

Consider, for example, the Prison Ships Martyrs Monument in Fort Greene Park, Brooklyn.

Erected by McKim Mead and White in 1908 to memorialize and house the remains of 11,500 Americans who died in prison ships during the Revolutionary War, it is a 149-foot-tall Doric column perched atop a 95-foot hill. Guarded by four 300-pound bronze eagles, and approached via a 100-foot-wide sweep of 99 stairs, it is magnificent, austere and about as conspicuous as a memorial gets.

Yet, as a memorial, it’s oddly invisible. People use the monument for skateboarding, picnicking, making out. They jog up the stairs in nice weather and sled down them in winter. What they don’t use it as is a mnemonic device. Few people know what it actually stands for, even when they are actually standing on it.

I know this because the memorial has recently taken on new significance for me. I was among scores of Brooklynites who climbed up to the monument’s platform and watched the twin towers burn and fall. We stood upon one mass gravesite and witnessed the creation of another.

In the intervening decade, that connection has intrigued me, and I’ve taken to quizzing people in the park about what they know of the bones under the hill. What I discovered is that 11,500 people who suffered agonizing deaths in service to their fledgling country have sunk into near-complete obscurity, despite the colossus that shouts their story from the summit, and despite the admirable efforts of a charming visitor center nearby. And so that 9/11 hilltop scene has come to represent a cruel and telling juxtaposition: Never Forget, and Largely Forgotten.

If you happen to be among those who have forgotten, allow me to introduce the martyrs.