Humans don't seem to be aware of it, but everywhere they go they leave a song. If you ask them about it they are genuinely ignorant of the phenomenon. Invariably attempts to repeat the tune back to them end in failure. Every species that encounters humans acknowledge it exists, but none can agree on just how it goes. Even beings with no auditory senses can hear it, describing the melody as a comforting feeling bouncing through their minds.

Telepathic races claim the music is likely some unconscious byproduct of chaotic human thought, more than a few overeager mind probers have gone catatonic looking too closely at a human mind so the topic is largely taboo among them.

When my people received an invitation to send a representative to the human home world of Earth for something called a "Remembrance Ceremony", I accepted as a senior member of the diplomatic core.

I had seen a hundred worlds and made a thousand journeys through the stars, but I never tired of the first view of a new planet when approaching orbit. Earth was beautiful, its blue oceans and green continents shrouded in white clouds. Here and there storms raged or the blazing lights of a metropolis shone from benighted areas.

As our shuttle crested the horizon my eyes fell on an enormous impact crater that seemed to be our destination. My data link pinged me with relevant information. The crater was roughly 700 km across, the result of an orbital impact some four centuries ago, causing a global extinction event. Such an impact would have rendered Earth a frozen and lifeless rock. I knew the dark circumstances of the crater, the calamitous introduction humanity had to a cruel universe, as did every other species who knew them. Our approach to the landing pad took us over great fields of obsidian and spires of rock. Here and there the shattered metal bones of buildings jutted from the ground where they had impacted after being thrown into the atmosphere.

A human was waiting at the bottom of the boarding ramp for me, so they could shepherd me to the meeting area when we exited. My species was not psychic but ever since we had entered the crater I had been feeling a deep sense of sorrow growing in my mind. As I walked with the guide the feeling was gradually replaced with the comforting melody my people felt in proximity to humans. I felt that I could almost hum the tune but it was somehow just barely out of reach, like a thought on the tip of the tongue. When we entered the open air plaza in the center of the crater I saw tens of thousands of humans and myriad beings from across the galaxy gathered.

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In the center of the plaza stood a beautiful obsidian monolith, 30 meters tall with an ornate carving of perhaps 200 humans holding hands and staring at the sky on its surface.

I could not make out the words but there was microscopic text filling the entire interior of the carving. I was just about to edge closer to make out what it said when the human speaker took her position at the podium.

"Friends from across the stars, we welcome and thank you for accepting the invitation to the Fourth Centennial remembrance ceremony. On this day we honor the 4.5 billion people that perished when the asteroid struck our home. The 4.5 billion people struck down by the malice of a greedy few who coveted what we did not even know we had."

Since the speaker had started, the ever present tune had faded somewhat, become more muted. I looked over the somber faces of the assembled humans, all were focused on the monolith behind the speaker.

"Behind me stands the monument to the fallen, carved from the obsidian blood of our wounded world. Etched into its surface is the name of every human that died as a consequence of the attack and in the effort to build our shelters to ride out the storm of its impact, as well as the name of every species we could not save, plant and animal alike."

As the speaker continued the song changed, I felt waves of nostalgia for my own home. Childhood friends and lost companions flashed through my mind, trips to the shimmer grass fields with my brood mother long passed. Though I had stood on the surface of many alien worlds, never had I felt such longing for my own.

"Though our world even today remains divided into many countries of many beliefs we remember that 195 nations, symbolized by the 195 people on the obelisk, laid aside their differences to assure a future for all of humanity. When we reach to the stars now, we do so with a clear purpose. We stride the stellar paths seeking to make sure no world suffers as Earth did."

The song had changed again, I felt now the comforting melody my kind were accustomed to. As the speaker continued the music drowned out her words, tens of thousands of humans on the plaza were singing the same tune together even as they stood in complete silence. Billions of humans across the planet and in the stars were serenading the universe with their hopes as they watched the ceremony with tranquil attention.

The xeno ambassadors throughout the crowd were looking around awestruck at the ranks of humans singing together in thunderous silence. Each heard it in a different way, but the feelings conveyed were the same.

Humanity was singing a song with their very souls. Each one ventured into the universe buoyed by hopes and a longing to make friends, to explore places no being had seen. They wanted to live their lives to the fullest, surrounded by friends and allies they made along the way.

It was the most beautiful music I had never heard.