The story describes a journey to the ocean bed in a specially-designed metal sphere; the explorer within discovers a civilization of human-like creatures.

From 1872 to 1876, HMS Challenger surveyed and explored around the world; the physical and chemical qualities of the deep sea were investigated, and biological samples were collected by dredges.

The explorer makes observations through the window in the sphere, oxygen inside being replaced by a fictitious "Myers apparatus".

On the sea bed, after observing the unusual species of fish and invertebrates, he sees "a strange vertebrated animal....

The narrator has talked to eminent scientists, who think such a civilization is quite possible; he further speculates: "We should be known to them, however, as strange, meteoric creatures, wont to fall catastrophically dead out of the mysterious blackness of their watery sky.