The Merman's Children.
"...this world...might soon crack to an end, not only in its present and future but in its very past." (p. 52)
Why does Tauno think this? If an event that might happen soon does in fact happen, then it will happen in our immediate future but it has not happened in our past. It does not make sense to say that an event might soon happen yesterday. See previous discussion of this issue here.
The mermen are about to descend into the mid-Atlantic depths but we have already discussed this process here.