The same thought was in Margaret's mind as she gazed frowningly forward; nevertheless, she managed to put some lightness into her tone as she spoke:
'I hope to goodness it's not another fall—I'm still feeling a bit sick from the last. I always thought that the people who go over Niagara in barrels were fools; now I know it.'
Mark drew her back from the windows.
'Better be on the safe side,' he said, pointing towards the seats.
The walls of the cavern closed in to leave a channel sixty or seventy feet wide. The Sun Bird was swept helplessly towards it, swinging and bobbing like a cork. Both held their breath as the course narrowed. It seemed impossible that they could safely clear the jutting spur at the left of the entrance. The stream gripped them, thrusting forward as if it could know no greater joy than to crush this metal eggshell upon sharp, savage rocks. Then, at the last moment, an outswirl of current deflected them. They were rushed past the spur with only a few inches to spare.
The searchlight had been left switched on, but it served only to heighten the effect of confusion. The beam flung erratically from side to side, confounding all sense of direction and giving every moment the impression that they were about to crash against one side or the other. Half a dozen times ominous scrapes on bottom or sides set their hearts pounding. One jolt after another convinced Mark that they were holed at last, but after each he was able to look round the cabin and see with relief that there was still no sign of leakage. It was a sharp cry from Margaret which managed, after twenty or more apprehensive minutes, to direct his attention farther ahead.
'Look!' she cried. 'Light!'
He stared through the forward windows. The motion made it difficult to be certain, but he seemed to catch a glimpse of a small patch of grey.
'Reflection,' he suggested.
'No, it's quite different. A colder, bluer kind of light than ours.'
He looked again, more carefully. The luminosity showed now in the shape of a high, almost Gothic, arch. 'As though,' he told himself, 'the end of this tunnel were silhouetted.' And its colour was, as Margaret had said, blue compared with the searchlight. Phosphorescence? But why should there be sudden phosphorescence? There was no trace of it in the water about them; a puzzle indeed, but soon to be solved, for their pace showed no sign of slackening.
He had to watch the opening grow larger and nearer for a longer time than he had expected. Distance in this darkness was deceptive, and his desire to be clear of the immediate dangers of the tunnel seemed to increase it. By the>time they swished out upon a second lake, he felt that miles had been covered. They left their seats when the former jerky progress subsided into a smooth gliding, and stood close against the windows. The silence of astonishment was broken by Margaret.
'It's impossible! I don't believe it.'
Mark, too, felt that the scene was more like a dream than a reality.
They were gazing across a lake which filled one of the largest caverns he had ever seen. So large was it that he looked apprehensively above; it seemed incredible that such a span of rock could be sustained without the help of pillars. But the size of the place, the acres of subterranean water accounted for only a small part of their astonishment. The phenomenon at which they stared open-mouthed was a system of lighting, beyond any doubt artificial.
At regular intervals about the roof were set globes which had the appearance of being frosted glass. From each came a glow of soft intensity, a light which was blue-white, yet not dazzling.
Mark's earlier anxiety was supplanted by a fresh nervousness. The lights had been erected for some purpose. But what purpose? And by whom? Hitherto they had faced natural, and roughly calculable dangers. All had depended on the Sun Bird's ability to survive; with her they had stood to live or perish. But with the discovery of the lights, a new element entered. They had been carried up against the unknown and, as always when all preconceived likelihoods are flouted, trepidation came crawling in. Men, he told himself, had put those lamps there—that could not be doubted. But what kind of men? What were men doing in these deep sunk caverns? Moreover, these glowing globes were unlike anything he had ever seen before—there was an entirely unfamiliar quality in the light they shed. As far as he could tell, they were a discovery not known above. What sort of reception might await intruders upon men who had for some reason hidden themselves deep and unsuspected in the earth? He glanced at Margaret, troubled. She was no longer gazing at the lamps; her attention had turned to the walls.
'Look, Mark,' she said, 'caves, there, above the water-line.'
He followed her pointing finger to see several openings, some near the water, others high in the wall, close to the roof. Moreover, he caught a detail which had escaped her. From the mouth of the largest a line of shadow ran slanting down into the water. A casual glance suggested a crack in the rock face; a longer look abolished the notion that it could be accidental.
'It's a ramp,' he said. 'A path leading up to that cave....'
For a few seconds he hesitated. They were drifting slowly now, and their course would take them not far from the wall. A disinclination to leave the comparative safety of the Sun Bird fought with the idea of a possible way back to the surface. To stay on board meant that they would allow themselves to drift farther and deeper into this maze of caverns. Already an unknown number of hundreds of feet lay between them and the daylight, and there was no guarantee that the exit from this lake would not take them over another fall. On the other hand, who—or what—might they encounter in the caves?
It was a choice of unknowns, but with the balance slightly in favour of exploration. After all, they could moor the Sun Bird, and have her ready for retreat. If they were to go on, there might be no other chance of landing. He pulled off his coat, and sat down to loosen his shoes, giving instructions to Margaret as he tugged at the laces.
'I'm going to swim over to that ramp. I want you to get on top and throw me a line when you're near enough. The control wires we used for mooring should be long enough if you join them. Think you can manage?'
She nodded, and started to search for the wires. Mark opened the door and dived out without hesitation, within ten minutes the manoeuvre had been accomplished. Mark had caught the curling wire, and the Sun Bird's metal belly was grating noisily as he pulled her bows on to the ramp. Margaret sprang down and stood beside him, watching him make fast.
'Isn't it quiet?' she said unhappily. 'I didn't think anywhere could be so horribly silent.'
Though she spoke in a low voice, the echoes managed to catch it and fling it eerily back and forth until it was no longer her voice, but a wandering, elemental sound. She shivered a little.
'I don't know which is worse; the silence, or the echoes.'
They listened for a moment to nothing. The silence sang in their ears with only an occasional clop-clopping of ripples to break it.
'Well,' said Mark cheerfully, 'as long as there's nothing but silence we needn't be afraid.'
The travesty which the jeering, booming echoes made of his statement seriously dismayed both of them. They glanced nervously at one another. Margaret took his arm, 'Do you really think we can get out that way?' she asked, looking up the ramp.
'Of course,' he managed with more conviction than he felt. 'The air's fresh here. There must be some way for it to circulate. If we can only-'
The rest of his sentence was drowned. There came the rumbling of a mighty crash, thunderous in the closed space. The solid rock beneath their feet trembled. Mighty reverberations like great breakers of sound buffeted back and forth across the cavern lake. A hundred yards along the wall a poised mass of rock detached itself and fell deafeningly into the water. Margaret's grip tightened on his arm. He could see her mouth forming inaudible words.