about ten to fifteen
Perpetual twilight reigned ; only a pale dim religious light filtered through . `` Just the thing , '' Wat exulted . `` We 'll never be found here , no matter how much they search , unless someone actually stumbles into the opening . There 's almost eighty feet of solid rock above us , and their search beams only penetrate about ten to fifteen . '' `` Splendid . '' Hilary said . `` Now we 've got to get to work . ''