When they were almost through the labyrinth, Van Emmon, after standing on tiptoes for the tenth time, in order to locate himself, noted something that had escaped their attention before.  “These compartments used to be covered over,” he said, for some reason lowering his voice.  He pointed out niches in the walls, such as undoubtedly once held the ends of heavy timbers.  “What was this place, anyhow?  A trap?”

Unconsciously they lightened their steps as they neared the last compartment.  They found, as expected, that it was another stairwell.  Van Emmon turned the light upon every corner of the place before going any further; but except for a formless heap of rubbish in one corner, which they did not investigate, the place was as bare as the rest of the floor.

Again they climbed, this time for a much shorter distance; but Jackson, slightly built chap that he was, needed a little help on the steep stairs.  They were not sorry that they had reached the uppermost floor of the mansion.  It was somewhat better lighted than the floor below, and they were relieved to find that the triangular compartments did not have the significant niches in their walls.  Their spirits rose perceptibly.

At the corner farthest from the stairs one of the walls rose straight to the ceiling, completely cutting off a rather large triangle.  The three paid no attention to the other compartments, but went straight to what they felt sure was the most vital spot in the place.  And their feelings were justified with a vengeance when they saw that the usual doorway in this wall was protected by something that had, so far, been entirely missing everywhere else.

It was barred by a heavy door.

For several minutes the doctor, the geologist, and the architect stood before it.  Neither would have liked to admit that he would just as soon leave that door unopened.  All the former uneasiness came back.  It was all the more inexplicable, with the brilliant sunlight only a few feet away, that each should have felt chilled by the place.

“Wonder if it’s locked?” remarked Van Emmon.  He pressed against the dust-covered barrier, half expecting it to turn to dust; but evidently it had been made of the time-defying alloy.  It stood firm.  And to all appearances it was nearly air-tight.

“Well!” said the doctor suddenly, so that the other two started nervously.  “The door’s got to come down; that’s all!” They looked around; there was no furniture, no loose piece of material of any kind.  Van Emmon straightway backed away from the door about six feet, and the others followed his example.

“All together!” grunted the geologist; and the three aluminum-armored monsters charged the door.  It shook under the impact; a shower of dust fell down; and they saw that they had loosened the thing.

“Once more!” This time a wide crack showed all around the edge of the door, and the third attempt finished the job.  Noiselessly—­for there was no air to carry the sound—­but with a heavy jar which all three felt through their feet, the barrier went flat on the floor beyond.