Our geologist explains why gravity is zero here. Because while one side of the Earth is pulling us that way, the other side is pulling in the opposite direction.

I ask him why people aren't pulled to one side? This is how he explains it. Start by keeping things simple: just consider a fixed proportion of the Earth on each side. (We'll scale up to the full Earth in a minute).

Every ton of the Earth's crust has the same pull (if you're the same distance away)

And if you are twice as far, the pull is half as much

But that fixed proportion now includes twice as many tons of rock. Result: exactly the same force

Scale it up to 360 degrees and the same applies in all directions. So wherever you are, the pull is the same in all directions, so you feel weightless!

Meanwhile, the Earth is spinning, so everything on the Earth feels a gentle push away from the north-south axis. This is what keeps everything sticking to the inner surface of the planet.

Of course, the bigger question is, why don't the seas drain into the hole at the top?

The scientist thinks that the top only opened temporarily, due to an Earthquake. At first water would rush in for a few hours. Or if the sea was shallow there, a few days. But this was the pole, remember, so there are gigantic icebergs nearby. Moving the water would cause some of it to break up, float toward the hole, and block it again.

At the time we thought the hole must be five hundred miles wide. This is because we assumed the horizon was flat and everything seemed so calm. In fact the hole was probably less than a mile wide. and we were sliding on a vast amount of gravel and ice.

Once icebergs block the hole the water will stop falling in. But small gaps will allow a determined party of sailors to carry a small boat back to the surface.

This just leaves three questions:

1. Why is the Earth hollow in the first place?

2. What prevents rocks from falling in and filling it up? Over millions of years the Earth would fill up, crumbling inwards.

3. How do we account for evidence that seems to contradict a hollow Earth? Such as how sounds from Earthquakes spread to the opposite side?

Here are some possible answers:

A) Perhaps the laws of nature are not what we thought?

B) Perhaps the laws of nature have changed? This is the "Schrodinger's Planet" theory. It extends the "Schrodinger's cat" theory. Before we test the inner earth, two sets of laws might be correct. Once we test it, only one set of laws is possible. So at the moment we test it, two parallel worlds separate: one where a hollow Earth is possible, and one where it is not.

C) Higher intelligences (aliens etc.) are behind this. They wish to be left alone, so they hide their presence. They are more advanced than us, so they can easily predict what tests we will make, and adjust the planet's crust so that it gives the results we expect.

This is a lot to take in! Should I take a break and explore the topic of higher intelligences? Or maybe explore the local islands to learn more about inner-earth geology? Or sail toward the nearby landmass to find the people who live here? Or do something else?