"We return greetings!" said Professor Stevens. "To whom, might I ask, do we owe our lives, and the honor of this interview?"

Larry smiled. The old graybeard was up to his form, all right!

"You are addressing Herr Rolf von Ullrich," the flattered German replied, adding genially: "commander of one of His Imperial Majesty's super-submarines during the late war and at present Emperor of Antillia."

To which the professor replied with dignity that he was greatly honored to make the acquaintance of so exalted a personage, and proceeded in turn to introduce himself and party. But Von Ullrich checked him with a smile.

"The distinguished Professor Stevens and his charming daughter need no introduction, as they are already familiar to me through the American press and radio," he said. "While as for Mr. Hunter, your Captain Petersen has already made me acquainted with his name."

At the mention of the commander of the _Nereid_, all three of them gave a start.

"Then--then my captain and crew are safe?" asked the professor, eagerly.

"Quite," Von Ullrich a.s.sured him. "You will be taken to them presently. But first there are one or two little things you would like explained--yes? Then I shall put to you a proposal, which if acceptable will guarantee your safe departure from my adopted country."

Whereupon the German traced briefly the events leading up to the present.

During the last months of the war, he had been placed in command of a special U-boat known as the "mystery ship"--designed to resist depth-charges and embodying many other innovations, most of them growing out of his own experience with earlier submarines.

One day, while cruising off the West Indies, in wait for some luckless sugar boat, he had been surprised by a destroyer and forced to submerge so suddenly that his diving gear had jammed and they had gone to the bottom. But the craft had managed to withstand the pressure and they had been able to repair the damage, limping home with a bad leak but otherwise none the worse for the experience.

The leak repaired and the hull further strengthened, he had set out again. But when in mid-Atlantic the Armistice had come, and rather than return to a defeated country, subject possibly to Allied revenge, he had persuaded his crew to remain out and let their craft be reported missing.

What followed then, though Von Ullrich masked it in polite words, was a story of piracy, until they found by degrees that there was more gold on the bottom of the ocean than the top; and from this to the discovery of the sunken empire where he now held reign was but a step.

They had thought at first they were looting only empty temples--but, finding people there, had easily conquered them, though ruling them, he admitted, was another matter. As, for instance, yesterday, when the priests had interfered with his orders and carried his three chief captives off to sacrifice.

"Where now, but for me, you would be food for their G.o.ds!" he ended.

"And if you do not find my hospitality altogether to your liking, friends, remember that you came uninvited. In fact, if you will recall, you came despite my explicit warning!"

But since they were here, he told them, they might be willing to repay his good turn with another.

Whereupon Von Ullrich launched into his proposal, which was that Professor Stevens place the _Nereid_ at his disposal for visiting the depths at the foot of the plateau, where lay the capital of the empire, he said--a magnificent metropolis known as the City of the Sun and modeled after the great Atlantean capital, the City of the Golden Gates, and the depository of a treasure, the greedy German believed, that was the ransom of the world.

The professor frowned, and for a moment Larry thought he was going to remind their host that this was not a treasure hunt.

"Why," he asked instead, "do you not use your own submarine for the purpose?"

"Because for one thing, she will not stand the pressure, nor will our suits," was the reply. "And for another, she is already laden with treasure, ready for an--er--forced abdication!" with a sardonic laugh.

"Then have you not enough gold already?"

"For myself, yes. But there are my men, you see--and men who have glimpsed the treasures of the earth are not easily satisfied, Professor. But have no fear. You shall accompany us, and, by your aid, shall pay your own ransom."

Von Ullrich made no mention of the alternative, in case the aid was refused, but the ominous light Larry caught in his cold gray eyes spoke as clearly as words.

So, since there was nothing else to do, Professor Stevens agreed.

Whereupon the audience terminated and they were led from the presence of this arrogant German to another apartment, where they were to meet Captain Petersen and the crew of the _Nereid._

As they proceeded toward it, under guard, Larry wondered why Von Ullrich had even troubled to make the request, when he held it in his power to take the craft anyway.

But after the first joyful moment of reunion, it was a mystery no longer, for Captain Petersen reported that immediately upon their capture, the commander of the U-boat had tried to force him to reveal the operation of the _Nereid_, but that he had steadfastly refused, even though threatened with torture.

And to think, it came to Larry with a new twinge of shame, that he had suspected this gallant man of mutiny!

That very morning, while Professor Stevens and his party were still exchanging experiences with Captain Petersen and the members of the crew, Von Ullrich sent for them and they gathered with his own men in the small lock-chamber at the base of the pyramid.

There they were provided with temporary suits by their host, since their own--which they brought along--could be inflated only from the _Nereid_.

Beside her, they noted as they emerged in relays, the U-boat was now moored.

Entering their own craft, they got under way at once and headed swiftly westward toward the brink of the plateau. Most of Von Ullrich's crew were with them, though a few had been left behind to guard against any treachery, on the part of the now sullen and aroused populace.

Slipping out over the edge of that precipitous tableland, they tilted her rudders and dove to the abysm below.

Presently the central square of the illuminated panel in the navigating room showed three great concentric circles, enclosed by a quadrangle that must have been miles on a side--and within this vast sunken fortress lay a city of innumerable pyramids and temples and palaces.

The German's eyes flashed greedily as he peered upon this vision.

"There you are!" he exclaimed, quivering with excitement. "Those circles, that square: what would you judge they were, Professor?"

"I would judge that originally they were the ca.n.a.ls bearing the munic.i.p.al water supply," Martin Stevens told him quietly, suppressing his own excitement, "for such was said to be the construction of the City of the Golden Gates; but now I judge they are walls raised on those original foundations by the frantic populace, when the submergence first began, in a vain effort to hold back the tides that engulfed them."

"And do you think they are of gold?"

"Frankly, no; though I have no doubt you will find plenty of that element down there."

Nor was the prediction wrong, for modern eyes had never seen such a treasure house as they beheld when presently the _Nereid_ came to rest outside that ancient four-walled city and they forced their way inside.

Though the walls were not of gold, the inner gates were, and the temples were fairly bursting with the precious metal, as well as rare jewels, the eyes of a thousand idols gleaming with rubies and emeralds.

But where was the populace, amid all this prodigious wealth? Was there no life down here?

Von Ullrich declared through the vibrator of his pressure-suit that he had heard there was. And as though in substantiation, many of the temples showed the same bell-jar construction as the pyramids above, though even stouter, revealing evidences of having been occupied very recently; but all were flooded and empty. The city was as a city of the dead.

This ominous sign did not deter the "emperor," however. Ruthlessly he and his men looted those flooded temples, forcing Professor Stevens and his party to lend aid in the orgy of pillage.