What's in Naboo's core, liquid water or plasma?

Question

One of the many things that bothers me about The Phantom Menace is the part where Obi-wan, Qui-gon, and Jar-Jar drive through 'the planet core'. I assumed that this meant that they literally drove through the center of the planet to arrive on the other side, meaning that the center of Naboo is filled with relatively average water. Which makes no sense, I would've expected some sort of superdense, superhot substance that you can't drive a submarine through.

However, a few answers on this site have pointed out that the location of the final fight with Darth Maul was a 'plasma refinery complex' that supposedly drew 'rare plasma' from the planet's core. Which would imply that Naboo's core is actually full of plasma instead of water.

It seems to me like the two cannot be true at the same time, you can't expect to see plasma and water existing simultaneously under the same conditions. So is one of these things a lie, or is this another case of Star Wars not using real science?

Answer 1

According to the "Star Wars Encyclopedia", the core is solid.

Naboo was a geologically unique world. It lacked a molten core, indicative of an ancient world.

Leland Chee also addressed this in Star Wars Insider 134

Q. How come the Jedi and Jar Jar can cruise through the center of Naboo's planet core in The Phantom Menace? Surely that would mean moving through lava?

Naboo is geologically unique because it lacks a molten core. The watery core of Naboo is known as The Abyss, a labyrinthine system of seas and waterways that begin on Naboo's surface and permeate its core. Though much of the wall is placid and cool, there are dangerous pocket currents created when fiery blasts from the planet's core heat the water to high temperatures. Extreme caution must be used when traveling in the honeycomb-like passageways of the inner planet.

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The 'Inside the Worlds of Star Wars: Episode One' factbook tells how eons ago the last remnants of "plasmatic magma" moving around inside the planet formed deep and long waterways in the bedrock, before finally dissipating into the rock itself. These are the underwaterways that the Gungans refer to as the "planet core". They don't strictly speaking travel through the entire core (e.g. from one side to the other) but they do burrow into the core material.

The Gungans farm 'locap' plants that extract (and stabilise) the plasma trapped inside the bedrock, which they then harvest for sale to the Naboo and offworlders. The Naboo use a different technology to attract and harvest what's left of the plasma energy in the rocks below Theed.

LOCAP FARMING

Locap plants grow very slowly and live for hundreds of years. Their roots bore into porous rock and siphon out plasma, which is naturally stabilized by the plants' digestive processes. This safe form of plasma collects in buds located at the tips of stalks. The Gungans farm plasma from the buds using harvesting subs. This is-a dangerous job, as locap buds have circular lobes with spiny teeth that snap shut when any pressure is felt.

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NABOO UNDERWATERWAYS

Underneath the surface of Naboo, a tremendous maze of passages and caves, created by movements of unstable plasmic energy in the interior, provides the Gungans with transport routes through the planet. Navigating these underwaterways requires immense skill since they are home to ferocious creatures. A single wrong turn can spell certain death. In spite of the risks, fleets of Gungan trading subs constantly voyage through these routes; overland travel on fambaas or kaadu is slower and more arduous for this amphibious species.

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Answer 2

A more recent (and canonical) answer comes from 2016's Star Wars: Complete Locations. It's a compilation of a lot of previously published material, including the images and text in the other answer here. However, there's a bit of extra information on page 16:

Naboo is a bizarre, geographically (sic) unique world located along a major galactic trade route near the Outer Rim. The planet's serene surface of sweeping hills and rolling seas is deceptive: beneath it lies a shadowy underwater world of winding caverns and tunnels inhabited by gigantic, ferocious sea creatures. This immense labyrinth runs through the entire planet, becoming increasingly rocky and dense in the lower strata. The planet's center seethes and bubbles with eruptions of exotic plasmic energy. Over millennia these eruptions form new caverns and tunnels, and influence surface features such as mountains. The planet's two primary civilisations, the Naboo and the Gungans, rely on this plasma power, although they collect and use it in different ways.

I suspect "geographically unique" was meant to be "geologically unique".

This does seem to say that the centre of the planet consists of plasma, not magma. Why that may be is not stated directly, but later in the book while discussing the power station of Theed (where Obi-Wan and Qui-Gon fought Maul), we do get this mention on page 55:

Mining Plasma: Deep within Naboo, plasma is constantly generated by natural ionization reactions.

Pretty vague. However, there's another nugget that seems rather intriguing, back on page 18:

The diverse terrain of Naboo teems with luscious plant and animal life. Whether it's the rich, green plains and insect-ridden swamps of the surface or the dark, fish-rich depths of the oceans, Naboo offers fruitful existences to those inhabitants who live in harmony with the planet. Several Gungan and Naboo chroniclers have advanced the theory that the mysterious and now long-dead civilisation whose ruins dot the countryside failed to respect this balance and were destroyed by their greed.

Now, Speculation alert! this suggests to me that the mysterious civilisation in question may have deliberately engineered the planet's core to produce plasma, and that engineering was what destroyed them. It's certainly not explicit, but it would explain the unique strangeness of Naboo.