Seas of Molten Rock
The floor really is lava on some worlds, which are covered by burning oceans of molten rock.
Take 55 Cancri e, a super-Earth orbiting a main-sequence star 41 light years away, for instance. It’s tidally locked to its star, so one side of the planet is permanently scorched by stellar radiation at a range of just 2.24 kilometers, with its surface temperature averaging over 4500⁰F (2480 C). At that temperature, the surface on 55 Cancri e’s dayside would be entirely molten – an entire hemisphere of lava ocean. The planet was once thought to be carbon-rich, like WASP-12b, but it turns out that 55 Cancri e’s burning ocean may be plain old silicate lava, not liquid diamonds. Even so, it would be a great place to play “the floor is lava,” or maybe stage a fight to the death with your former Jedi apprentice.
So far, scientists haven’t found a good explanation for that. The night side, meanwhile, is a still-scorching 2010⁰F (1100 C), cool enough to have solid ground underfoot and perhaps even a little bit of atmosphere. According to Spitzer’s thermal mapping, a significant amount of that heat probably comes from the inner workings of the planet itself, which is unusual for a tidally-locked world, where tectonic activity should long since have ground to a halt. Scientists still haven’t found a good explanation for that, but it’s possible that it could be a hint of other exoplanets in the 55 Cancri system.
55 Cancri e isn't the only lava world in the universe, either, so you have another option for that Jedi death match. 480 light years away in the constellation Monoceros, the super-Earth CoRoT-7b is tidally locked to its yellow dwarf star, which it orbits at a close range of just 1.5 million miles. CoRoT-7b's neighbors help keep it active despite the tidal lock; tidal forces from two other exoplanets in the system keep the planet's interior hot and moving, so the planet's 4000⁰F (2200 C) dayside, much like that of 55 Cancri e, is an ocean of liquid rock. The night side is a much cooler -350⁰F (-212 C), but still sees constant volcanic eruptions. .
Some of the greatest adventure stories on Earth take place on the high seas, often in the blank spots on the map, marked only "Here Be Dragons." A map of the galaxy, however, offers even wilder oceans to explore, and someday bold adventurers might sail on seas of lava, plasma, tar, methane, or strange supercritical fluids.
A few of these bizarre oceans might even harbor life in strange forms we can only speculate about for now. Maybe there are microbes living in the tarry seas of a carbon-rich world around Wasp-12, or maybe some future mission will encounter methane-dwelling sea monsters on Titan. One thing is certain: the universe is a wet, wild place, and there are far, far more than seven seas to explore.