Over a couple of years, the dome did not only become a hiding place, but slowly the mermen left their caves and started building houses.
At first only a couple of houses, since many still feared the volcano and its powers.
But gradually the few houses grew to become a town.
The mermen experienced their Industrial Revolution. No more threatened by predators, they started using their brains to improve their living conditions.
And one of them was to be able to move the city to more prosperous places.
Even though the design was originally organic, in the end the new city became a magnificent piece of technology.
And when the mermen had finally grasped the mechanics of flying and how to create a closed habitat, nothing was to stop them anymore.