An unexploded bomb believed to be around 300 years old has been found under the Old Riding House at the Akershus Fortress in Oslo, Norway.

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Experts say the bomb could date from military exercises carried out by King Christian IV of Denmark in the 17th century, or a siege by King Karl XII of Sweden in 1716, Aftenposten reported. Norway was part of Denmark until the early 19th century and was then joined to Sweden until it gained independence in 1907.

"This is an extremely rare and very interesting find," said Britt-Alise Hjelmeland, who is in charge of antiquarian maintenance at Akershus.

Hjelmeland said melting at the sides of the 1-inch powder hole suggests the bomb had been fired.

The bomb was found about 3 feet under the floor of the Old Riding School, which was built in 1828 near the former artillery workshop. Harold Moberg, the fortress' curator, said he believes it was test-fired.

"The bomb can have lain unused, gotten mixed up in road material and been forgotten," Moberg said. "If it had been fired in attack it would probably have been destroyed."