Sen became the first Indian on record to swim across the Palk Straits on 5–6 April 1966 between Ceylon (Sri Lanka) and Dhanushkodi (India) in 25 hours and 36 minutes. On 24 August, he became the first Asian to cross the Straits of Gibraltar (Europe to Africa) in 8 hours and 1 minute, and on 12 September became the world's first man to swim across the 40-mile long Dardanelles (Gallipoli, Europe to Sedulbahir, Asia minor) in 13 hours and 55 minutes. In the same year, Sen was also the first Indian to swim the Bosphorus (Turkey) in 4 hrs and the first non-American (and third man) to swim across the entire (50-mile length) of the Panama Canal in 34 hrs and 15 mins on 29–31 October.