Pufferfish World

It almost seemed like a punishment, at the height of humanity's greatest point in steam-based technology it all fell apart. We don't know if any empire was to blame, perhaps it was divine punishment for our sins, or a natural phenomena, but in the end it tore us apart; the pufferfish. From the ocean floor bloomed with millions upon millions of these creatures rose to the surface, covering every square inch it in pufferfish. No ship could sail the seas without it's hull town apart by the torrent of the sharp toxic barbs leading to the collapse of the world's greatest naval empires. And for nearly 700 years humanity was alone, isolated, and starved as without international trade society as we knew it collapsed, the era of colonialism was over. Humanity is crafty and these pufferfish will not stop everyone. Floating cities and crawlers are a testament to humanity's endurance, massive steam powered sovereign metropolises with legs to walk across the ocean floor between islands, safe from the pufferfish above.