Opposite of "miracle"?

Disaster is very close in root meaning to what you're looking for, describing as it does an "ill-starred" event.

Debacle has the advantage of looking like an antonym to "miracle".

Miracle is still the right word. As Terry Pratchett puts it,

“Whatever happens, they say afterwards, it must have been Fate. People are always a little confused about this, as they are in the case of miracles. When someone is saved from certain death by a strange concatenation of circumstances, they say that's a miracle. But of course if someone is killed by a freak chain of events: the oil just spilled there, the safety fence just broke there : that must also be a miracle. Just because it's not nice doesn't mean it's not miraculous.”

“Act of God” is sometimes used in English law for an event that is not reasonable to predict, for example flooding in a area that does not normally get flooding.

Consider force majeure ("an overwhelming force" or "an unavoidable catastrophe") and act of God ("an unforeseen occurrence beyond one's control, such as a natural disaster"). Also consider slightly-related idioms or phrases like chain of events, road to ruin, snowball, runaway train and booby trap ("an unforeseen or unexpected or surprising difficulty").