The Pentagon has decided to lay an estimated $40 million underwater fiber-optic cable from Guantánamo Bay to South Florida, The Miami Herald has learned, in the latest sign that the military is preparing for detentions and other operations at the Navy base for the long-term.

“It only makes sense to do if we’re going to be here for any period of time,” said Navy Capt. Kirk Hibbert, disclosing the project in an interview last week before ending a two-year tour as the Navy base commander.

Construction won’t start for more than a year. And communications won’t come online for probably two more years

But the American military has already notified the Cuban military to expect a surveyor ship, the USNS Zeus, off the base’s coastline this summer — a first step toward getting the program funded and then out to bid.

The fiber-optics plan is the largest known infrastructure improvement for the base by the Pentagon, which has undertaken expansion and building projects in a mostly piecemeal and sometimes secretive fashion in the decade of housing war on terror captives there.

Army Lt. Col. Todd Breasseale said the Defense Information Systems Agency had done a “feasibility study” and put the tentative price tag at $40 million. It will require congressional approval, he said, and is in the fiscal 2013 budget.

At Guantánamo, Hibbert said increasing data delivery from the base, which has both the war court and the prison camps intelligence unit, had stretched satellite access from the outpost and planners studied whether to expand their “terrestrial system,” or go under the water with fiber optics.

Plus, satellite links are prone to interference during bad weather, when the century-old outpost may need connectivity the most. The base, population about 6,000, is like a small town with a seaport, airport and the detention center that houses 169 foreign men as captives, with 1,700 troops and contractors on temporary assignment to imprison them.

The Heritage Foundation’s Cully Stimson, who was in charge of detainee policy for the Department of Defense during the administration of President George W. Bush, said the investment makes short-term sense with the coming war crimes trials, notably of the five accused Sept. 11 plotters. But he warned that the investment in the infrastructure does not necessarily signal that the Pentagon is now preparing for detention of prisoners at Guantánamo forever.

“That naval station’s been around since 1903, and it will live long past the detention mission,” said Stimson, now Heritage’s chief of staff and senior legal fellow. “It may be a fiscally prudent use of taxpayer funds.”

The Pentagon also uses the 45-square-mile base as a contingency site for humanitarian relief operations. It has fields prepared to house in tents thousands of people who might flee social unrest or natural disaster in the Caribbean, as refugees from Cuba and Haiti did in the 1990s.

Even if President Barack Obama were to succeed in his ambition to close the detention center, Stimson said, the infrastructure there could be put to other use.

Maintaining Guantánamo is expensive, and the constant churn of prison staff adds to the cost. Navy Cmdr. Tamsen Reese, the recently departed public affairs officer, said the prison estimates it costs taxpayers $77 a day to house and feed a soldier or sailor assigned to detention center duty.