March glanced at the Skipper and saw that he was looking at a huge panel on one wall. Yes, this was the “Christmas Tree!” It was a large electric indicator board covered with red and green lights. It showed the exact condition of every opening—hatches, air induction vents, and all—into the ship. Everything having anything to do with diving had its indicator there on the board. March saw that most of the lights were green, but many were still red. He knew that every light had to be green before the ship could dive.
“Stand by for diving,” said the Skipper in a quiet voice.
Sutherland, standing behind him, sang out, “Stand by for diving!” The telephone orderly repeated the order over the interphone to all parts of the ship and March jumped as the klaxon horns blared out their raucous warnings. For a moment their sound reverberated in the small steel room, and then Sutherland barked new orders.
“Open main ballast Kingstons!” March saw the men move the levers as he repeated the order, and a few lights turned to green on the “Christmas Tree.”
“Stop main engines!” The order was repeated over the phone to the engine room. March felt the trembling of the ship stop as the Diesels were shut off and the electric motors switched on again, taking their current from the huge banks of storage batteries under the deck of the ship. At the same time other lights turned to green on the board.
“Open main ballast vents!” called Sutherland.
One after another the necessary orders were called by the diving officer, they were carried out with precision and reported back at once. Finally, the last red lights on the board winked out as the main air induction valves were closed. Then Sutherland ordered, as the last test, that air be released from the high-pressure tank into the interior of the ship. March watched him look at the dial indicating air pressure within the ship. The hand moved up a little, then held steady. This showed that there was no leakage of air from the boat.
Sutherland turned to the Skipper. “Pressure in the boat—green light, sir.”
“Take her down!” said the Captain with a nod.
When the diving officer repeated the order the klaxons blared again their final warning before the diving officer called out one order after another. March had been able to keep close track of everything up to this point, but suddenly, just at the crucial moment, there was too much going on. He heard an order that sounded like “Down bow planes!” and felt the ship tip forward slightly. But at the same time he heard the roar of water as it rushed into the ballast tanks between the inner and outer steel hulls of the ship, the rush of air forced out of the vents by the inrushing water, and the whine of the electric motors.