 What caused the explosion that sunk the Elena Mia?
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. None, to the best of his knowledge, was called Puerto Pacifico.
 An alien craft fired on the Elena Mia from under the water.
 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.