Next weekend I will be running another short adventure in my Flood campaign world, and the adventure trigger will be the arrival near the Hulks of one of the drowned earth’s most potent environmental threats: the Miasma.
The Miasma is a special type of jellyfish swarm that can only exist in the depopulated aquatic deserts of the world ocean. With almost all land sunk thousands of metres below the surface of the ocean there are very few areas where fish can spawn and thrive, and huge expanses of the earth’s surface are too far from these spawning areas for larger fish to be able to survive in them. These areas, too far from fish populations to support any biodiversity larger than plankton, have become a strange and top-heavy ecosystem. Aside from occasional large predators migrating through, these empty wilds have become the domain of the grazers: whales, basking sharks and jellyfish that thrive on the plankton in surface waters. But the most efficient and terrifying of these open ocean grazers are the jellyfish, which sometimes by happenstance gather together into huge swarms, sometimes tens of kilometres across, that dominate the open ocean wherever they drift, and leave a terrible path of destruction behind them, like enormous army ants of the open seas.
The survivors of human civilization live in terror of these jellyfish swarms, which they call miasma. These swarms, though composed primarily of plankton-eating drifters, are also usually heavily infested with giant, slow-moving grazers and a large number of deadly predatory jellyfish. They consume everything in the ocean around them, and are an existential threat to the fishing grounds that most human communities zealously nurture. They also transform the atmosphere and sea around the swarm, turning the ocean into a kind of limpid swamp that offends the sensibilities of drifting raft communities, and is deeply toxic.
Most raft communities float on oceans more than 1000 m deep, and are as vulnerable to the currents as a jellyfish swarm. To maintain a reliable ocean food supply, the rafters carefully and systematically build up local ecosystems, which in turn feed a network of wild distant ecosystems. In areas like the Gyre, where a relatively large number of human communities live in close proximity, this produces a wider network of ecosystems that spread beyond the immediate vicinity of the rafts and support wildlife somewhat akin to the offshore ecosystems of the old world. But these systems are fragile, and require careful stewarding by the rafters. Every human community that floats on the waves has some system of undersea support for local fish life, that may be as simple as a series of sub-surface floating breeding beds made of old tires, or a complex architecture of reed beds and corals under the hulls and keels of the rafts. In the Gyre, where human communities are structurally large and complex, there are a host of shallow underwater structures that are thriving with sea life that once would have only existed at the shoreline: lobsters in nooks and crannies of the rafts, oysters growing on the old submarine superstructure that gives the larger rafts stability, and small fish breeding amongst every chain link, tyre underside, and submerged rope knot in the entire archipelago of floating human life. There is even a small community of sea lions on the Hulks. Every underwater structure is covered in sea weeds, and kelp grows downward and outward from the edges of the rafts. Further out, larger fish prey on the smaller fish, moving nomadically between communities and eating the larger fish that live further out from the human settlements. The residents of the Hulks put a lot of time into the care of these undersea communities, enforcing strict waste disposal rules and carefully tending weedbeds and corals to ensure that the ecosystem is balanced and thriving.
A miasma can end 100 years of this careful management and stewarding in a few days of insensate gorging. When the miasma overwhelms a human community it will consume everything that floats free in the water. All plankton and feeder fish will be sucked up by the filter feeders, and the larger fish will be found and killed by the predatory jellies. Sea mammals in the open water when the miasma hits will be entangled and drowned, or slowly paralyzed by the accumulation of stingers. The water the miasma carries with it is devoid of oxygen and highly acidic, and if the miasma is moving too slowly on the currents the effect of this tide of pollution will be to destroy all corals and weeds in the raft and its vicinity. Lobsters, crabs, prawns and octopuses that might be untouched by predatory jellies will suffocate in the miasma, and once the swarm has passed all that remains will be dead and rotting sea life. Sometimes the miasma does not pass, and the weight of its central parts will drag the raft community with it, leading to starvation and death for all those onboard. If the rafters do not realize the danger, escape may be impossible: small sailing ships do not have the power to escape the weight of jellyfish in the water, and most smaller powered vessels will have their rudders and rotors entangled, stalling them in the water. Once trapped in the sea of deadly stingers, there is no way to swim out. There are many stories of miasmas that are haunted with the ghosts of lost rafts and ships, and many people claim that they are more dangerous for a small raft community than the ocean’s storms.
The miasma carries its own meteorological phenomena, that in large part are the reason for its name. On the edge of the swarm freely moving predatory jellyfish hunt anything that moves, but in the body of the swarm are primarily filter-feeding drifters, some of them larger than a small boat. Near the centre of this gelid mass the sea becomes so thick with packed-together jellyfish that it is almost solid, and devoid of any other life. When the jellyfish here become too densely packed or when the swarm becomes too large, they die and rot, creating a fetid and stinking swamp of rot. The largest swarms becalm the sea around them, changing its texture so that waves are dampened and currents stop; there in the middle of this becalmed and dense swamp-like realm, the sun beats down and the sea heats up, giving off a stinking and rotten cloud of steam and heat haze that obscures the surface of the water. Most swarms above a certain size carry with them a thick, swamp-like haze that is impenetrable on all but the stormiest of days. Rumour has it that the largest swarms break up under their own meteorological effects: the heat from the centre creates storm cells that scatter the swarm and cast it about into smaller swarmlets. Usually one can smell the swarm before one sees it, because tendrils of this rot drift ahead on the currents. If one is lucky the swarm will be large enough and dense enough that the rafts can be moved out of the currents, or some kind of defenses prepared. But for the larger swarms, or for rafts adrift on a strong current, there is no defense: only prayer.
The miasma leaves problems long after its passing. In addition to destroying all marine life attached to the rafts, it will leave polyps on every surface, and every part of the underside of the rafts needs to be scoured clean to ensure that the raft does not, a year later, become the centre of its own swarm. Jellyfish will also destroy nets and befoul important undersea structures, their weight breaking precious ropes and chains, blocking inlets for power and water desalinators, and poisoning the water as they die and rot. Delicate floating solar panels will be damaged or lost, and the acidic water may do irreparable damage to the oldest and most fragile parts of a long-lived raft community. Small boats may be carried away under the pressure of the swarm, or their propellors and other underwater components entangled and ruined. The raft will also find itself floating in an open sea devoid of life, and any community that is used to catching fish in the immediate vicinity of its decks will need to find a way to venture further afield for food until the ecosystem rights itself – if ever it does. Most likely, though, the local ecosystem will be destroyed, and the rafters will need to make a call on other human communities they know to obtain new stocks of coral, shellfish and weed to regrow their precious ecosystem. Usually such help comes with a high price, that many communities cannot pay: they will then break up, and individual rafts will be forced to brave the open ocean as they seek new fishing grounds or functioning communities to join.
The larger communities are the most vulnerable, because they cannot move. These communities have found ways to ride out the miasma, or even to divert it, but their efforts almost always come with a great cost, in equipment and often lives. Where once jellyfish were annoying pests at the beach, they have now become the greatest terror of the open ocean, and for a community like the Hulks the onset of a major miasma is a threat that most of its residents will only ever see once in their lifetimes – if they survive it …