The Maracot Deep by Sir Arthur Conan Doyle

It was a few days after our arrival that our hosts or our captors -- we were dubious sometimes as to which to call them -- took us out for an expedition upon the bottom of the ocean. Six of them came with us, including Manda, the chief. We assembled in the same exit chamber in which we had originally been received, and we were now in a condition to examine it a little more closely. It was a very large place, at least a hundred feet each way, and its low walls and ceiling were green with marine growths and dripping with moisture. A long row of pegs, with marks which I presume were numbers, ran round the whole room, and on each was hung one of the semitransparent bells of vitrine and a pair of the shoulder batteries which ensured respiration. The floor was of flagged stone worn into concavities, the footsteps of many generations, these hollows now lying as pools of shallow water. The whole was highly illuminated by fluor tubes round the cornice. We were fastened into our vitrine coverings, and a stout pointed staff made of some light metal was handed to each of us. Then, by signals, Manda ordered us to take a grip of a rail which ran round the room, he and his friends setting us an example. The object of this soon became evident, for as the outer door swung slowly open the sea water came pouring in with such force that we should have been swept from our feet but for this precaution. It rose rapidly, however, to above the level of our heads, and the pressure upon us was eased. Manda led the way to the door, and an instant afterwards we were out on the ocean bed once more, leaving the portal open behind us ready for our return.

Looking round us in the cold, flickering, spectral light which illuminates the bathybian plain, we could see for a radius of at least a quarter of a mile in every direction.