The Persians put out word that any Greek who revealed information helpful to them would be rewarded.   One Greek, who desired the reward, told the Persians about the location of a secret path through the mountains.  Above the pass of Thermopylae, this path was known to local people only.  The path is shown on the map above by a dotted line.  The path would allow the Persians to come secretly through the mountains and around behind the Greek army guarding the pass below. The Greeks would then be trapped with the Persians in front of and behind them.