Complex sentence: They reacted by first making the mariners take a blood alcohol test, which came up negative: there really was a white whale swimming in the Rhine 300 kilometers from the ocean and thousands of kilometers from the usual habitat of the beluga whale in arctic waters.
Split into simpler sentences: They reacted by first making the mariners take a blood alcohol test. There really was a white whale swimming in the Rhine 300 kilometers from the ocean and thousands of kilometers from the usual habitat of the beluga whale in arctic waters.