When fighting erupted in Gaza last month and rockets rained on Israel, the men operating its navy submarine Leviathan knew next to nothing.
Submerged for a drill, they were relayed only brief bulletins so as not to be distracted from their mission.
"We are by definition clandestine and, to a degree, isolated at sea. Our performance depends on our focus," a senior navy officer told Reuters aboard Leviathan during a training run, in the first such foreign media access to the vessels.
Speculation surrounds the role of the fleet of five Dolphin-class submarines, each costing some $500 million. A sixth is on order from the German manufacturer. Some analysts suggest these boats may be capable of launching nuclear missiles.
Around a third the size of the nuclear-powered giants of the United States or Russia, and with diesel-electric engines that limit underwater durations to two or three weeks, the Dolphins are designed mainly to patrol the Mediterranean coast.
But in 2019 Prime Minister Benjamin Netanyahu also tasked the navy with tackling arch-enemy Iran.
In January, a Dolphin made a rare visit to the Red Sea, where several Iranian-linked ships have been sabotaged.
Leviathan's below-decks weapons section was off-limits to the press. So was discussion of actual operations.
Swinging the periscope to peer at cargo ships above, the captain ordered a deeper dive and a simulated attack. A control-room display showed Leviathan's torpedo tubes as "flooded", or ready for launch.
The floor tipped, the control-room crew leaning against the incline. A few dozen meters down, Leviathan switched from diesel to electric power. To preserve the resulting quiet, crewmen passed along targeting and fire orders in murmurs, reinforced by finger taps on comrades' backs.
A speaker aired the torpedo's sonar signal: like the song of a cricket, rising in urgency then falling silent as the target was "sunk".
A Dolphin's core operational crew size is 45, with an average age of 22, the senior officer said. Ten or more people can squeeze aboard, for training or deployment as frogmen.
At their homeport of Haifa, a fortified hangar shields the Dolphins from rocket strikes or hostile eyes. Crews are expected to put to sea at extremely short notice, the officer said.
Leviathan is named after the biblical whale. Dining, as elsewhere in the IDF, is kosher. That means separate kitchenware for meat and dairy which, on Leviathan, resulted in spillover cutlery being stored in the corridor.
Prayers precede Sabbath meals, with blessings over grape juice instead of wine. Purified seawater allows for regular showers and laundry by hand. A static bike, board games and videos pass downtime.
Cramped quarters mean junior submariners rotate three-to-a-bed. On longer trips, some wear casual clothes, and everyone goes by first names except the captain and first mate.
The Dolphins are among a handful of Israeli military units whose personnel must forfeit any other citizenships they hold, a precaution against pressure to spy for foreign powers.
There is almost no "compartmentalization" during missions. The crew are kept apprised of secret plans to create a sense of common cause, the senior officer said.
Some analysts speculate the submarines may serve as so-called "second strike" platforms capable of launching nuclear missiles autonomously, a deterrent against a surprise attack.
"It would be far more difficult to be sure of destroying submerged submarines," said Norman Friedman, a scholar with the U.S. Naval Institute. "If Israel were to deploy a submarine-launched missile, I would put my money on a cruise missile."
Israel, which does not confirm or deny having nuclear arms, aims to prevent Iran from getting such weaponry. Iran says its nuclear designs are peaceful. It also calls for Israel's demise.