https://underfootnotes.com/Clear answers about the ground we walk on, from pebbles to plate boundaries.https://underfootnotes.com/cores/the-map-that-read-the-rocks-how-william-smith-charted-the-layers/https://underfootnotes.com/cores/the-map-that-read-the-rocks-how-william-smith-charted-the-layers/Izaiah BarionnetteTue, 01 Sep 2026 00:00:00 +0000A canal surveyor noticed that rock layers stack in a fixed order and carry their own fossils, and used that to colour the first geological map of a whole country in 1815. Here is how the idea grew, what it got right, and why geological maps still work the same way.https://underfootnotes.com/cores/how-did-we-learn-the-earth-has-a-core-we-have-never-seen/https://underfootnotes.com/cores/how-did-we-learn-the-earth-has-a-core-we-have-never-seen/Priya KettlewellSat, 27 Jun 2026 00:00:00 +0000Nobody has ever drilled anywhere near the Earth's centre, yet we know it has a liquid outer core and a solid inner one. The answer is a century of detective work, using the way earthquake waves bend, slow down and disappear on their way through the planet.https://underfootnotes.com/cores/a-slinky-a-rope-and-a-stone-seeing-how-earthquake-waves-move/https://underfootnotes.com/cores/a-slinky-a-rope-and-a-stone-seeing-how-earthquake-waves-move/Lucinda MarakiSun, 24 May 2026 00:00:00 +0000Earthquakes send out two kinds of waves: one pushes and pulls, the other shakes side to side. Only the first can cross a liquid, and that difference is how we learned the Earth has a liquid outer core.https://underfootnotes.com/cores/from-trapped-winds-to-springy-rock-how-we-learned-what-shakes-the-grou/https://underfootnotes.com/cores/from-trapped-winds-to-springy-rock-how-we-learned-what-shakes-the-grou/Izaiah BarionnetteTue, 31 Mar 2026 00:00:00 +0000An earthquake is slowly squeezed rock suddenly slipping and sending waves outward. It took people a few centuries, plenty of wrong turns and some patient measuring to see it that way. Here is the story, and the physics behind that first thump and the sway that follows.https://underfootnotes.com/cores/why-do-volcanoes-line-up-in-rings-and-chains/https://underfootnotes.com/cores/why-do-volcanoes-line-up-in-rings-and-chains/Moana AldridgeThu, 19 Feb 2026 00:00:00 +0000Volcanoes aren't scattered at random. Most sit where one tectonic plate dives beneath another, or where a plate slides over a long-lived source of heat. Here is the step-by-step picture, with a little arithmetic and a few honest "we're not sure yet" moments.https://underfootnotes.com/cores/why-does-water-expand-when-it-freezes-and-what-does-that-do-to-rock/https://underfootnotes.com/cores/why-does-water-expand-when-it-freezes-and-what-does-that-do-to-rock/Rewi HallworthFri, 16 Jan 2026 00:00:00 +0000Water is one of the few common substances that takes up more room as a solid than as a liquid. That oddity is why a forgotten jar splits in the freezer, and why frost slowly pries cliffs, roads and mountain slopes apart.https://underfootnotes.com/cores/why-a-pebble-feels-smooth-when-its-parent-rock-was-rough-how-we-learne/https://underfootnotes.com/cores/why-a-pebble-feels-smooth-when-its-parent-rock-was-rough-how-we-learne/Izaiah BarionnetteWed, 19 Nov 2025 00:00:00 +0000A pebble is smooth because every knock against another stone takes off its sharpest parts first, and there are a great many knocks on the way down a river. Getting people to believe that slow wearing could explain the rounded stones in their own valleys took geologists a surprisingly long time.https://underfootnotes.com/cores/how-did-people-figure-out-the-earth-is-billions-of-years-old/https://underfootnotes.com/cores/how-did-people-figure-out-the-earth-is-billions-of-years-old/Moana AldridgeSat, 13 Sep 2025 00:00:00 +0000Nobody measured the Earth's age with a single clever test. It took centuries of looking at rock layers, one wrong-but-careful estimate, and the discovery that atoms keep time. Here's how the pieces fit together.https://underfootnotes.com/cores/why-do-rocks-come-in-three-families/https://underfootnotes.com/cores/why-do-rocks-come-in-three-families/Priya KettlewellSun, 13 Jul 2025 00:00:00 +0000Every rock you pick up has a life story, and nearly all of them fit one of three plots: cooled from melt, pressed from settled bits, or changed by heat and squeezing. Here's how to read the clues in your own hand, and why the "families" are really one long loop.https://underfootnotes.com/cores/the-continents-that-wouldn-t-sit-still-how-wegener-s-idea-was-eventual/https://underfootnotes.com/cores/the-continents-that-wouldn-t-sit-still-how-wegener-s-idea-was-eventual/Izaiah BarionnetteThu, 12 Jun 2025 00:00:00 +0000A century ago, a meteorologist suggested that the continents had once been joined and slowly drifted apart. He was mostly right, but it took decades, and a lot of ocean-floor mapping, for the idea to catch on.https://underfootnotes.com/cores/what-is-actually-under-your-boots-a-layer-by-layer-walk-down/https://underfootnotes.com/cores/what-is-actually-under-your-boots-a-layer-by-layer-walk-down/Priya KettlewellMon, 02 Jun 2025 00:00:00 +0000Start with the grass or gravel under your feet and keep going, through soil, rock, crust, mantle and core, all the way to the centre of the Earth. Along the way: how we know what's down there when nobody has ever been anywhere near it.https://underfootnotes.com/cores/why-is-the-sea-salty-when-the-rain-is-fresh/https://underfootnotes.com/cores/why-is-the-sea-salty-when-the-rain-is-fresh/Priya KettlewellMon, 07 Apr 2025 00:00:00 +0000Rain tastes of nothing much, rivers taste of very little, and yet the sea is properly salty. The answer lies in what a raindrop picks up on its way down and what evaporation leaves behind.