Henry Molaison (HM) who was 27 years old went to get a bilateral medial temporal lobectomy to surgically resect the anterior two-thirds of his hippocampi because he was suffering from epeleptic seizures. He had Dr. William Scoville perform his surgery which was the removal of his hippocampi.
Everything seemed normal until they had realized that HM was unable to form or gain any new memories of his own. They had sent him back home to his parents house and had sent different psychologists to check up on him.
Later on in 2008, when he passed away his brain was donated to science and was dissected into 2401 pieces. This allowed researchers to study what had happened to HM over the course of 50 years. after the dissection of the brain researchers had also found that Scoville had also damaged HM's Amygdala which plays a vital role in controlling of emotions. After dissection scientist had come to the conclusion that the hippocampus plays a vital role in creating and storing memory. That is why when HM had removed it he had lost the ability to store memories.