The way the water just filled in the crater from the explosion. Estimates of the size of the explosion seem to vary, but in general they put it as equivalent to a small nuke, with 0.2 kilotons at the low end and as much as 3 kilotons at the highest end. I'm inclined to think it was around 1 kiloton, but even the smallest estimates are still a ridiculously large explosion. Just horrible. So many things had to have gone wrong for this to have happened, so many mistakes had to have been made. Lebanon hasn't gotten a break in the last half-century.