Seoul, South Korea (CNN) -- Korean is considered one of the hardest languages in the world to master, but an elephant in a South Korean zoo is making a good start.

Koshik, a 22-year-old Asian elephant has stunned experts and his keepers at Everland Zoo near Seoul by imitating human speech. Koshik can say the Korean words for "hello," "sit down," "no," "lie down" and "good." His trainer, Kim Jong Gap, first started to realize Koshik was mimicking him several years ago.

""In 2004 and 2005, Kim didn't even know that the human voice he heard at the zoo was actually from Koshik," zoo spokesman In Kim In Cherl said. "But in 2006, he started to realize that Koshik had been imitating his voice and mentioned it to his boss."

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His boss initially called him "crazy."

Koshik's remarkable antics grabbed the interest of an elephant vocalization expert thousands of kilometers away at the University of Vienna in Austria.

""There was a YouTube video about Koshik vocalizing, and I was not sure if it was a fake, or if it was real," Dr. Angela Stoeger-Horwath said. She traveled with fellow expert Dr. Daniel Mietchen to South Korea in 2010 to test the elephant's ability. They recorded Koshik repeating certain words his keeper said and then played them for native Korean speakers to see, if they were recognizable.

"It is, for some of the sounds he makes, quite astonishing for how similar they are," said Mietchen of the University of Jena in Germany. "For instance the word 'choa' (meaning good) -- if you hear it right after what the keeper says -- it's quite similar."

1. What grabbed expert's interest?
1. unknown