There is a bizarre rocky landscape, well hidden from our prying eyes, in the secretive shadows under the oceans of our Earth. Here, in this strange and alien domain, it is always as dark as midnight. Thin, tall towers of craggy rock emit billows of black smoke from their peaks, while all around the towers stand a weird, wavy multitude of red-and-white, tube-like organisms--that have no eyes, no intestines, and no mouth. These 3-foot-long tubeworms derive their energy from Earth itself, and not from the light of our nearby Sun--a feat that most biologists did not believe possible until these wormish creatures were discovered back in 2001. The extremely hot, superheated black water, billowing out from the hydrothermal vents erupting on Earth's seafloor, provides high-energy chemicals that sustain the tubeworms, as well as other weird organisms that apparently thrive in this very improbable habitat.
Vast regions of dark dunes also extend across Titan's exotic landscape, especially around its equatorial regions. Unlike Earth's sand, the "sand" that creates Titan's dunes is composed of dark grains of hydrocarbon that resemble coffee grounds. The tall linear dunes of this misty moisty moon-world appear to be quite similar to those seen in the desert of Namibia in Africa. Because Titan's surface is pockmarked by relatively few impact craters, its surface is considered to be quite young. Older surfaces display heavier cratering than more youthful surfaces, whose craters have been "erased" by resurfacing. This resurfacing is caused by processes that cover the scars left by old impacts as time goes by. Our own planet is similar to Titan in this respect. The craters of Earth are erased by the ongoing processes of flowing liquid (water on Earth), powerful winds, and the recycling of Earth's crust as a result of plate-tectonics. These processes also occur on Titan, but in modified forms. In particular, the shifting of the ground resulting from pressures coming from beneath (plate tectonics), also appear to be at work on this veiled moon-world. However, planetary scientists have not seen signs of plates on Titan that are analogous to those of our own planet.
Europa: Planetary scientists generally think that a layer of liquid water swirls around beneath Europa's surface, and that heat from tidal flexing causes the subsurface ocean to remain liquid. It is estimated that the outer crust of solid ice is about 6 to 19 miles thick, including a ductile "warm ice" layer that hints that the liquid ocean underneath may be 60 miles deep. This means that Europa's oceans would amount to slightly more than two times the volume of Earth's oceans.