They were within the dam. They were in a pit of smooth, blank, perpendicular sides; there was nothing to afford even the slightest handhold; and no exit save the overhead slit. It was a part of the mechanism's internal, hydraulic system.
o Larry's horror he soon discovered that the water was slowly rising! It was breast-high to him now, and inch by inch it crept up toward his chin. It was already over Tina's depth: she clung to him, half-swimming.
Larry soon found that there was no possible way for them to get out unaided, unless, if they could swim long enough, the rising water would rise to the height of the aperture. If it reached there, they could crawl out. He tried to estimate how long that would be.
"We can make it, Tina. It'll take two hours, possibly, but I can keep us afloat that long."
But soon he discovered that the water was not rising. Instead, the floor was sinking from under him! sinking as though he were standing upon the top of a huge piston which slowly was lowering in its encasing cylinder. Dimly he could hear water tumbling into the pit, to fill the greater depth and still hold the surface level.
With the water at his chin, Larry guided Tina to the wall. He did not at first have the heart to tell her, yet he knew that soon it must be told. When he did explain it, she said nothing. They watched the water surface where it lapped against the greasy concave wall. It held its level: but while Larry stood there, the floor sank so that the water reached his mouth and nose, and he was forced to start swimming.
Another interval. Larry began calling: shouting futilely. His voice filled the pit, but he knew it could carry no more than a short distance out of the aperture.