On the side of the valley in which the first complex is found, there is a giant skull carved into the overlooking crag. It’s easy—given the other transgressions in the film—to dismiss this as spookhouse attempt at being scary. But what if (stay with me here) it’s a warning sign, an alien Mr. Yuk, put there for other sentient humanoids to understand that this place is deadly with a capital D? This explains why the outpost hasn’t been disturbed by rescuers of their own race. They were smart enough to see the warning and turn right back around. (Why they didn’t nuke it from orbit is another question.)

The Material

Seeing this as a warning label raises other questions. Why wouldn’t a warning be technological or linguistic, like most of the interfaces inside the complex? The black infection material is still deadly after 2000 years. Who knows how much longer it will be viable? So where the interfaces inside are for immediate use, the warning outside needs to be effective for millennia, outlasting both the power reserves that would drive technology and the persistent semantics that would cement linguistic understanding. Rock, in contrast, lasts a very, very long time. Even during the erosion the shape and its clear meaning will simply lose clarity, not wink out altogether.

The symbol

Similarly, this shape is a clear symbol of death that is tied to biology, which changes on evolutionary timeframes, guaranteeing its readability for—hopefully—longer than the xenomorph liquid would be a danger.

For these reasons, this is labeling that is more than a Castle Grayskull set dressing attempt at scaaaarrrry, but a reasonable choice at providing an effective warning that will last as long as the danger. You know, providing visiting scientists actually pay attention to such things.