"Well, it was just an accident," Sally admitted. "You see, Genevieve and I haven't much to do most of the time but roam around by ourselves, so we've managed to poke into most of the places along the shore, the whole length of this river, one time and another. It was last fall when we discovered this. We'd climbed down here one day, just poking around looking for beach-plums and things, and right about here I caught my foot in a vine and went down on my face plumb right into that lot of vines and things. I threw out my hands to catch myself, and instead of coming against the sand and dirt as I'd expected, something gave way, and when I looked there was nothing at all there but a hole. "Of course, I poked away at it some more, and found that there was a layer of planking back of the sand. That seemed mighty odd, so I pushed the vines away and banged some more at the opening, and it suddenly gave way because the boards had got rotten, I guess, and--I found _this_!" Doris sighed ecstatically. "What a perfectly glorious adventure! And what did you do then?" "Well," went on Sally simply, "although I couldn't make very much out of what it all was, I decided that we'd keep it for our secret,--Genevieve and I--and we wouldn't let another soul know about it. So we pulled the vines and things over the opening the best we could, and we came up next day and brought some boards and a hammer and nails--and a candle. Then I fixed up the rotten boards of this opening,--you see it works like a door, only the outside is covered with vines and things so you'd never see it,--and I got an old padlock from Dad's boathouse and I screwed it on the outside so's I could lock it up besides, and covered the padlock with vines and sand. Nobody'd ever dream there was such a place here, and I guess nobody ever has, either. That's my secret!" "But, Sally," exclaimed Doris, "how did it ever come here to begin with? Who made it? It must have some sort of history." "There you've got _me_!" answered Sally. "Some one must have stayed here," mused Doris, half to herself. "And, what's more, they must have _hidden_ here, or why should they have taken such trouble to keep it from being discovered?" "Yes, they've hidden here, right enough," agreed Sally. "It's the best hiding place any one ever had, I should say. But the question is, what did they hide here for?" "And also," added Doris, "if they were hiding, how could they make such a room as this, all finished with wooden walls, without being seen doing it? Where did they get the planks?" "Do you know what that timber is?" asked Sally. "Why, of course not," laughed Doris. "How should I?"