The greatest of all depths of which we have reliable evidence was found by the _Challenger_ on the 23d March, 1875, in the comparatively narrow channel which separates the Caroline Islands from the Mariana or Ladrone Islands. This sounding amounts to 4,575 fathoms, or about five miles and a quarter. Several soundings exceeding 4,000 fathoms were obtained by the _Tuscarora_ to the eastward of the islands of Nippon and Yesso, and another close to the most westerly of the Aleutian Islands. Two of these soundings are over 4,600 fathoms, but as it appears that no sample of the bottom was brought up, there is no evidence of the latter having been reached. H.M.S. _Challenger_, shortly after her departure from Yokohama, sounded 3,950 and 3,625 fathoms, and in doing so seems to have just touched the southern border of this deep but narrow area of depression, which runs parallel to the eastern coasts of Japan and the Kurile Archipelago as far as the entrance to the Behring Sea. It will be observed that the above exceptional depths in the Atlantic, Indian, and Pacific Oceans are not placed, as one might be inclined to conjecture, in or near the centre of these oceanic basins, but, on the contrary, upon their confines and in close proximity to the land. This remarkable circumstance suggests the idea that such areas of maximum depression may be the effect of a sinking of the bottom of the sea in compensation for an upward movement of the land in their immediate vicinity. Just as the results of the recent soundings have rendered the existence of depths from six to nine miles, as formerly reported, highly improbable, so have they modified our ideas of the shape of the sea-bottom. The latter was generally represented as a repetition of the dry land with its combination of mountain, valley, and plain. No doubt the sea-bottom within a short distance of the shore naturally forms a continuation of the leading features of the adjoining land. Thus a large plain or a low-lying country will, as a rule, continue its almost level slopes to a considerable distance out to sea, while a range of hills or a chain of mountains often extends its steep inclines below the surface of the water.