In October, 1588, the ship San Juan De Sicilia landed in Tobermory Bay, on the Isle of Mull, Scotland.  The ship was a unit belonging to the Squadron of Levant of the Spanish Armada.  The Spanish ship was heavily damaged by English ships and the crew was short of water. The captain arrogantly demanded aid from the locals. 

Local chieftain, Lachlan MacLean of Duart and the Spanish captain negotiated for supplies.  The deal was that the captain could have as much food as he liked but he must pay for it in gold.  MacLean came to his aid with the promised food, and the Spanish captain gave MacLean 100 men with the agreement that the Spanish troops would help settle some local feuds with nearby clans; clan MacIan of Ardnamurchan and clan MacDonald of Clanranald.  The Spanish agreed and MacLean and his newly acquired mercenaries set out to attack the local clans. When the MacIan and MacDonald reinforcements arrived, MacLean and the Spaniards withdrew their burning and sacking throughout the region.

The Spanish captain soon announced to MacLean that he was ready to sail.  The Spaniard said he would pay in gold only when his men were returned to his ship.  MacLean agreed but kept three Spanish officers as hostages while waiting for payment in gold.  MacLean sent his own son to the galleon to collect the gold.  Once on board, the son was taken prisoner and the Spanish began to set sail despite leaving their officers behind.  A short time later, the bay was rocked with a huge explosion and the gold filled galleon sank to the bottom of the sea. 

Is it possible the ship was destroyed by an English spy?  Or could the clans MacIan or MacDonald have organized the dastardly deed in seeking revenge for the sackings?  It is highly unlikely Lachlan MacLean would blow up his Spanish allies that took his own son!  No one will ever know!

Resources:

http://www.thesonsofscotland.co.uk/thespanisharmadainscotland.htm