question:  What caused the explosion that sunk the Elena Mia?
context: Opening the desk drawer, which was almost empty, Pembroke removed an automatic pistol fitted with a silencer. The series of events leading up to Pembroke's present occupation had commenced on a dismal, overcast evening in the South Pacific a year earlier. Bound for Sydney, two days out of Valparaiso, the Colombian tramp steamer Elena Mia had encountered a dense greenish fog which seemed vaguely redolent of citrus trees. Standing on the forward deck, Pembroke was one of the first to perceive the peculiar odor and to spot the immense gray hulk wallowing in the murky distance. Then the explosion had come, from far below the waterline, and the decks were awash with frantic crewmen, officers, and the handful of passengers. Only two lifeboats were launched before the Elena Mia went down. Pembroke came as close to being a professional adventurer as one can in these days of regimented travel, organized peril, and political restriction. Lacking such loyalties, Pembroke adapted quickly to the situation in which he found himself when he regained consciousness. His situation could have been any small western American seacoast city. One in particular, an attractive woman of about thirty, tossed back her long, chestnut locks and gazed up intently at Pembroke as he passed. "You're not casual enough, for one thing," said Pembroke, deciding to play along with her for the moment. Also you're a bit knock-kneed, not that it matters. Pembroke watched with lifted eyebrows as the clerk whisked the bizarre statuette underneath the counter. "Incidentally, just where are we, anyway? "Puerto Pacifico," she told him. But as he left the shop he began checking off every west coast state, city, town, and inlet. None, to the best of his knowledge, was called Puerto Pacifico. "Which way is it to San Francisco?" asked Pembroke. "This is Puerto Pacifico." "All cargo ships, I'll bet. No passengers," said Pembroke. Going to the bar, he recognized one of the Elena Mia's paying passengers.
output:  An alien craft fired on the Elena Mia from under the water.
options:  An alien craft fired on the Elena Mia from under the water.  A Colombian ship fired a torpedo on the Elena Mia, causing it to explode.  The Elena Mia ran into an iceberg. The ice pierced the ship's electronics causing an explosion.  The androids tampered with the Elena Mia's electronics causing an explosion.