Theseus came to Carboneras, Spain late last year. The mast is an extendable pole from which a fuel saving kite is flown. It's a complicated apparatus, and when it's in operation it looks like a huge parachute hovering about 150m above the vessel. Notice the two extra windows looking upwards from the central bridge windows. The Captain said that it really works, I think he said it gave something like 30% saving on fuel costs. He even had a minicam picture taken from the chute looking down to the vessel. Impressive stuff!