This is the dramatic moment tourists scream with fright when a giant manta ray swims under their boat.<br /><br />The holidaymakers had been on the speedboat visiting the Similan Islands in the Andaman Sea when the 18ft-long beast was spotted in the distance.<br /><br />The mammoth marine creature began splashing at the surface around 100ft away before moving alarmingly quickly towards the vessel's starboard side.<br /><br />Its huge body was seen as white outline lurking menacingly beneath the surface as it glided under the ferry and appeared on the port side.<br /><br />A second clip then showed the manta ray circle back around the front of the speedboat, splashing around and swirling its tail, before disappearing into the depths of the ocean.<br /><br />The heart-stopping footage was captured by a tour guide from Andaman Sea Sun Travel going to the popular Thai holiday islands on May 8. <br /><br />They said: ''We saw the manta ray and everyone was getting a little bit excited, including me. I've never seen one before so it was quite an amazing moment.''
