The country’s prime minister said the incident, the latest in a series of disruptions to undersea infrastructure, underscored the danger of a so-called Russian shadow fleet.The Finnish police on Thursday boarded an oil tanker that the authorities said they suspected was involved in damaging vital undersea cables. They suggested the tanker was part of a shadow fleet used by Russia to avoid Western sanctions.The Estlink 2 submarine cable, which carries electricity between Finland and Estonia, was cut on Wednesday, Finland’s police said in a statement, the latest in a slew of disruptions to undersea infrastructure that are being investigated as...