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 canals bearing the
municipal 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 th