We are so used to picturing us humans on Earth's surface (and maybe soon Mars) that when we see a planet like Jupiter, one of the first questions that comes to mind is "Can we stand on it?"
Let’s for a moment ignore the extreme conditions (gravity, atmospheric pressure, high temperature, and winds) that are found on the giant planet, and let’s just descend through the atmosphere. What we’d see is a spectacle like no other.
Far beneath Jupiter's atmosphere is a gigantic ocean of liquid metallic hydrogen, which would look and behave like mercury except that hydrogen has 60 percent the density of water. So, you would have to sink for tens of thousands of kilometers to reach a hot, molten, rocky core that's possibly solid.
The interior of Jupiter is not precisely mapped – that is one of the scientific goals of the Juno mission, which has just reached Jupiter. The probe will use precise gravitational and electromagnetic measurements to map what goes on underneath Jupiter’s clouds.
I know that the answer so far is not really satisfying, so let’s drop the pretense of a magical observer and let’s dive into Jupiter with current technologies.
Jupiter is really warm, with the top of its atmosphere at about 900 Kelvins (630°C / 1,160°F). As we fly down through the atmosphere, the temperature quickly drops as the pressure and wind speed increase. Following what happened to the Galileo probe, which dived into Jupiter in 1995, things start going badly for electronics 58 minutes into our exploration and 156 kilometers (97 miles) into the atmosphere. For the Galileo probe, this combination of pressure (23 atmospheres) and temperature (153°C / 307°F) proved deadly.
But it's been 20 years, so let's assume we can continue down. At 500 kilometers (310 miles), the visibility is almost completely gone and the thick ammonia clouds swirl all around us, with wind speeds of around 100 meters (330 feet) per second.
Underneath the ammonia clouds, there are more water clouds and more complex atmospheric effects that Juno will hopefully clarify. Current technology will have been pulverized by now, and our remains will now be traveling to a layer of supercritical fluid hydrogen – something not quite a gas but not exactly a liquid either.
After about 2.5 hours of exploration, we will have reached the liquid metallic hydrogen ocean. Heavy elements might reach the center after many more hours of falling. So, you couldn't exactly stand on this ocean. But below this, it's thought Jupiter may indeed have a rocky core, perhaps somewhat similar to terrestrial planets. Juno will help answer this question during its mission.
Jupiter has most of the planetary mass of the Solar System, and could easily fit all the other planets in its interior. There's a reason why it is the planetary king of the Solar System.