In the short story "Krakatau" by Jim Shepard, the last line quoted from a Krakatau witness was, "I would give all these people's lives, once more, to see something so beautiful again." This was an eyewitness account of the eruption and its aftermath. What is the significance of this line?
The significance of the quote is that it points to the devastating beauty that the eruption of Krakatau must have displayed. Its eruption is one of the ten largest volcanic eruptions in history. 200 megatons of destructive power were unleashed in 1883, which makes it four times more powerful than any nuclear bomb ever detonated. It's also the third deadliest eruption in history. Between the pyroclastic fallout and the massive tidal waves that the eruption generated, about 34,000 were killed. It must have been a truly amazing thing to have witnessed—terrifying, too, I'm sure. But it is one of those events in history that scientists (volcanologists, geologists, etc.) have studied quite extensively. I'm sure many of them would jump at the chance to go back in time and witness the event (with a guarantee they wouldn't be killed). Tidal gauges registered changes in ocean heights up to 7,000 miles away. Seeing that thing go off would have been nuts.
The quote reflects that sentiment. Some readers might interpret it as a morbid quote. A person wishes for all of those deaths again just to see a volcano go off? Yes, that's a morbid interpretation, because it focuses on the deaths. A slightly more positive interpretation is to focus on the beauty. Something must be absolutely awe inspiring in its beauty to even contemplate trading that many lives. That's how I see the line being significant. It's focused on the powerful beauty of that eruption.