Global sea levels are rising – there is no doubt about it. But what comes next? Some land near the coast is very likely to be flooded. Should we let it? Or should we try to build dams to keep the water out? We tried to answer this question by studying what happens when you flood uninhabited coastal land. Would it just turn into some sort of “underwater wasteland,” or into a functioning aquatic habitat that both animals and people can use? To find out, we followed the creation of the Gyldensteen Coastal Lagoon, an area in Denmark set aside to become a natural reserve, for two years. We conducted lab experiments and field observations to see how some marine bristle worms respond to flooding. We found that they did well, they changed the chemistry of their environment, and the newly flooded land developed into something resembling a functioning new marine ecosystem.