Sea water is salty and contains radioactive 40K (potassium-40). in this salt one can found The nucleus of the atoms of this Potassium decay into Calcium and an electron. This electron emits Cerenkov light when travelling through water and may fake a muon cerenkov hit.
The string of Test 1 was first deployed on October 6, 1996 and retrieved on October 8, 1996. From that time many others lines where immersed to get detailed results on this potential background.