Ukraine’s only submarine, the Zaporozhye, underwent first-stage sea trials including running submerged on Wednesday after undergoing a major refit, Kommersant Ukraine reported on Thursday.

The boat was submerged in the Black Sea for the first time in 18 years during the trial, Fleet Commander Vice Admiral Yury Ilyin said. The tests will continue until Friday.

Surface tests of the submarine’s power plant and other key equipment were held in April.

A Soviet-era Foxtrot class (Project 641) diesel-electric submarine, Zaporozhye was laid up at a dock in Sevastopol for years before its refit.

Russian engineers repaired and serviced the submarine and a new crew of Ukrainian submariners was trained at Russian naval bases.\

Though the submarine was built 40 years ago, it has been upgraded with new equipment.

Russia, Bulgaria, and Romania also have submarines based in the Black Sea.