WHY FOR ALL THE NOISE

This just happened maybe two minutes ago: I'm in my room getting ready to go to work when I hear basically all the sirens in the city go screaming past my house (the main road that winds through the Presidio passes not far from my back porch). It's not uncommon for me to hear a siren every once in a while; I assume they're ambulances or cops heading for accidents on the Golden Gate Bridge. But this sounded like a truly serious number of sirens. What the hell is going on out there?

I'm taking a different way to work today.