Submarines are quiet, deadly and expensive. Boats like those in the Virginia class, which is a U.S. attack submarine, can cost $3.4 billion and take seven years to build. The Navy has ambitious goals for the future of the underwater fleet, but some problems could stand in the way.”The Navy is undergoing a 20-year plan that will cost $21 billion to upgrade its infrastructure,” said Aidan Quigley, a reporter at Inside Defense who covers the U.S. Navy and Marines. “Right now, the state of Navy shipyard infrastructure is not great. They’ve been underfunded for the past couple of decades.”The Navy currently has 68 submarines in service. And it wants to start shipbuilding on two to possibly three Virginia-class attack subs per year, and roughly one Columbia-class submarine per year until around 2035. But according to the Congressional Budget Office, a lack of shipyard infrastructure could delay those plans.”The Navy is focusing on improving productive capacity via initiatives to increase on-time delivery and operational availability while reducing maintenance costs,” said Navy Lt. Rob Reinheimer, in a statement to CNBC.And in response to the Government Accountability Office report on Columbia-class procurement released in January, Reinheimer said, “Over the past three years the Navy, with strong Congressional support, has invested over $573 million in shoring up existing sources and development of new suppliers.”The recently released defense budget request for fiscal 2022 could be less than what the Navy needs to keep pace with China and Russia, according to some observers.Watch the video above to find out how the Navy will upgrade its multibillion-dollar submarine fleet.