Greek and international experts seeking the remains of a Persian armada that sank in northern Greece 2,500 years ago will press on with the project next year despite drawing a blank this summer, it was announced yesterday. A first underwater survey last autumn off Mount Athos - where the large invasion fleet came to grief during a storm in 492 BC - had come up with the encouraging find of a bronze spearbutt, in an area where fishermen had pulled up two 2,500-year-old bronze helmets in 1999. But a survey carried out between late May and mid-June failed to locate any more weapons or artifacts dating to the time of the invasion, Shelley Wachsmann of the Texas A&M University's Institute of Nautical Archaeology told an Athens conference on deep-water archaeology yesterday. Several wrecks were located, mostly dating to the past few centuries. The 2005 survey will focus on the southwest coast of Mt Athos. According to the historian Herodotus, the storm cost the Persians 300 ships and 20,000 men in the first of their three great expeditions against Greece.