However, the death of Gollum at the end of Return of the King got me thinking. Gollum, if you remember, dove into the lava of Mount Doom after his precious ring was thrown in — he proceeds to sink into the lava (see below) and leaves the ring floating on the lava until it melts away. Guess what? Sinking into lava just will not happen if you’re a human (or remotely human). You’d need to be a Terminator to sink into molten rock/metal … and here’s why.
Molten lava is nothing like water. Sure, everyone thinks that liquid rock (magma) is going to behave like any other liquid (e.g., water), but there are some key physical properties that tell us it just isn’t the case.
Then there's the math and an experiment that explains why one doesn't sink in lava. Still, you don't want to try it at home. Link -via Not Exactly Rocket Science