Shlomi Katzin attached a GoPro camera to his forehead, slipped on his diving fins and jumped into the waters off the Carmel coast of Israel, eager to go exploring.

On the sandy floor of the Mediterranean Sea, he found a sword. Archaeologists would later determine that it was about 900 years old.

It weighed four pounds, measured about four feet long and originated from the Third Crusade, experts said.

“Oh yes, he was surprised and happy,” said Jacob Sharvit, the director of the marine archaeology unit at the Israel Antiquities Authority.

Katzin said he would give the sword to Sharvit’s agency, but he wanted just one thing: a photo with the shell-encrusted weapon. The recent discovery was welcomed in a country that takes pride in its history and has a law requiring that any artifacts found must be returned to the nation.

The sword was among several artifacts discovered by Katzin, who declined to be interviewed because he said he did not want the discovery to be about him. He also found stone anchors and pottery fragments that date back hundreds of years. But nothing was more impressive than the sword, which Sharvit described as “extremely rare”.

All of the items were found in the same 1,000-square-foot site. The authority has been aware of the location since June, after a storm shifted the sand. Still, finding artifacts remains elusive because of the movement of the sand.

“It’s normal to find swords in bad condition, but this one was found under the water — and under the water, it was preserved in very good condition,” Sharvit said on Monday.

“It’s the first time that we found a beautiful sword like this.”The water off the Carmel coast remains the same temperature year-round, which helped preserve the iron in the sword. Because the iron was oxidised, shells and other marine organisms stuck onto it like glue, Sharvit said.