The Other Side (1970)

In the spring of 1994, a series of mysterious events occur in the small town of Stryhiltsi, Luhansk Oblast, Ukraine. While several of the town's children go missing, three teenage boys discover a local Soviet-blocked river, which appears to have the ability to send a diver 30 years into the future. When the boys try it out, they find themselves in the Russian-occupied Luhansk Oblast in the 2020s, where they find a connection between the local kidnapper and the "time machine" river.