This one started in a garden, with a seven-year-old and a perfectly reasonable question. My niece asked whether it was true that if she dug straight down she would end up in Australia. I told her it would be close, but probably somewhere between Australia and New Zealand, which is the sort of answer that satisfies most people. She was not most people. She pointed at the ground a few feet away and asked: "Where would that go to?"
I had no idea. I assumed someone must have built something to answer exactly this kind of question. What I found online was a lot of information about antipodes, which tells you what is directly opposite any given point on the Earth. Useful, but not the same thing. The antipode assumes you are drilling straight down. My niece was not asking about straight down. She was asking about a specific direction and a specific angle, and the honest answer was that I could not find anything that handled that properly.
So I started looking into whether it was actually possible to calculate this. It turned out not to be straightforward at all. The Earth is not a perfect sphere. The geometry involved in tracing an arbitrary path through it at a given pitch and bearing, and then working out where that path exits on the other side, is genuinely involved. I got far enough into it to understand the problem and then, as tends to happen, other things got in the way. The idea sat in the back of my mind for a few years.
When I was made redundant in September 2024, I decided to finally sort it. A few months later, EarthTunnelling.com was live. It is a mobile web app that lets you point your phone's camera in any direction and find out where a straight-line tunnel through the Earth in that direction would emerge. Not the antipode. Not straight down. The actual direction you are pointing, at the actual pitch your phone is held, translated into a specific point on the other side of the planet.
That is the core of it, but the app does more than that. You can map out multiple locations and calculate the weighted centroid, which is the geographical centre of a set of points. I used this to find the true centre of the Formula 1 world by plotting every circuit on the 2025 calendar, which turned out to be a good demonstration of what the feature can do. But the same idea works in a lot of directions. The centre of everywhere you have ever travelled. The geographical balance point of a group of friends spread across the world. The point where your travel history actually lives, rather than the places you just flew through. Every new location shifts it, which makes it the kind of thing you want to keep coming back to. There is also an interactive game mode that tests your spatial awareness in real time, which is harder than it sounds and considerably more addictive than it has any right to be.
Thinking about where this could go, a child at home pointing their phone at the ground and deciding that this year's holiday should be wherever the tunnel comes out is a fairly compelling way to pick a destination. It is also, arguably, a better decision-making process than most adults use. A frequent traveller watching the weighted centre of their journeys shift across the globe with every new trip is a very different way of thinking about where you have actually been. These are not features I have built yet, but they are the kind of thing this platform makes possible, and they are the reason I think there is more to this than a curiosity project.
The app works on both Android and iPhone, though it was designed primarily for Android and performs best there. There are four videos on this page covering the short promo, sensor calibration, a full introduction to the features, and the F1 centroid piece. If you want to understand what the app does and how to use it, the introduction video is the right place to start.