Harry's Island is the volcanic island in the underground sea where the expedition encounters a towering geyser, and its naming gives the travelers a fixed point in a journey otherwise measured across an immense, unfamiliar expanse. The island is at once a geological spectacle, a challenge to Professor Hardwigg’s theory about heat within the Earth, and a personal landmark named in honor of Harry.

Setting and Discovery

The island first presents itself at a distance as an enormous whale, its apparent head rising above the water. The shape proves deceptive: the supposed sea monster is an island crowned by a geyser. Professor Hardwigg recognizes the phenomenon and explains that geysers are among Iceland’s natural wonders, while the party’s approach transforms an uncertain silhouette into a display of concentrated volcanic power. The jet rises from the island’s summit; unlike the surrounding hot springs and vapors familiar from Iceland, this geyser stands alone, its plume illuminated by the electric light.

The landing requires care. Falling water could swamp the raft, so Hans steers it around the hazard and brings the travelers to the island’s farther side. Harry is the first to leap onto the rock; Professor Hardwigg follows, while Hans remains comparatively unmoved. The ground is granite mixed with siliceous sandstone and shivers beneath their feet. At the central basin, Harry measures the bubbling water at 163 degrees. The observation makes the island more than a dramatic sight: it supplies direct evidence of intense heat beneath the surface.

A Challenge to the Professor’s Theory

The water’s temperature appears to contradict Professor Hardwigg’s theory, and Harry tells him so. The Professor answers sharply, asking what the measurement proves against his doctrine. The exchange brings the novel’s tension between theoretical conviction and direct observation into a compact scene: the thermometer registers a striking fact, but the Professor refuses to treat it as decisive disproof. Harry, meanwhile, recognizes that the expedition may eventually reach regions where central heat exceeds anything their instruments can measure.

The island therefore marks a pause in the voyage without resolving the scientific dispute. Harry watches the geyser’s force ebb and surge, attributing its irregular rise to changing pressure in the vapor reservoir. The display invites observation and explanation, yet does not settle how the Earth’s interior works. The travelers leave with the question still open, carrying the island’s measured heat as a concrete complication in the Professor’s account of their surroundings.

Naming and Measurement

Before the expedition departs, Professor Hardwigg proposes naming the island and considers a name associated with his nephew. The naming transforms the stop into a landmark within the travelers’ own geography. Harry’s subsequent account records the practical measure that makes the place useful: since leaving Port Gretchen, the party has traveled 270 leagues—more than 800 miles—across the inland sea. At that observation point, they are 620 leagues from Iceland and directly beneath England.

After the storm, Professor Hardwigg calls the place Harry's Island and insists that Harry accept the honor of having named an island discovered by the expedition. The name thus records a relationship as well as a location. It also supplies a reference point for reconstructing the party’s course when the storm has made speed and direction difficult to determine. The landmark’s significance lies partly in this contrast: the travelers claim a place in a vast subterranean world by naming it, then use that name to recover their bearings in a voyage whose route has been violently disrupted.

Place in the Underground Journey

Harry's Island belongs to the expedition’s passage across the underground sea, but it also concentrates the journey’s recurring acts of observation. The travelers identify a distant form, correct a mistaken impression, land, inspect the rock, measure the water, debate what the measurement means, and enter the location into their own record of the voyage. Its name and coordinates make the unknown legible without making it fully understood.

The island’s two kinds of importance remain joined. As a volcanic site, it exposes the party to a source of heat intense enough to trouble Professor Hardwigg’s theory; as a named point on the route, it helps the travelers describe how far they have crossed and where they believe themselves to be. Harry's Island is consequently not just a picturesque interruption. It is a moment when empirical evidence, scientific interpretation, personal recognition, and the act of mapping meet—and when the expedition’s claim to knowledge remains provisional even as its map grows more exact.