Why not just put a bunch of spikes 50 feet below the ground, with a lot of scattered stones cut into the shapes of skulls. Most people will get that, and it will avoid having a prominent above ground landmark.
Although on the other hand, if there are above ground landmarks, then knowledge about what the place signifies is less likely to vanish.
Although on the other hand, if there are above ground landmarks, then knowledge about what the place signifies is less likely to vanish.