Anatomy of Swimming.
The sixth machine, and the third medium a body can move through — after air and ground, now water. A robotic fish, taken apart three ways: the body that swims, the one rule of moving through water, and the brain that sets its pace. Two pieces of physics none of the others have: buoyancy, so it holds depth for nothing, and quadratic drag, so the harder it pushes the less it gains. Every number on the board is computed by the Institute's open swim core, so the page cannot lie to you — and it completes the trio of media: air, ground, water.
Guess before you look.
Each opens with a question almost everyone gets wrong, a fish you can turn over in 3D, and the one move that makes it obvious. Open them in order, or jump to the layer you want.
What holds it at depth?
“A drone burned 200 W to hover. What does this robot spend to hold its depth?”
Almost nothing. Tuned to neutral buoyancy, the robot displaces exactly its own weight in water, so the upward buoyant force cancels gravity and the net vertical force is zero. The water does continuously what a drone's rotor must — for free.
Open the demonstration →Why doesn't it just go faster?
“It swims at one metre a second. Push twice as hard — how fast does it go?”
Only √2 ≈ 1.41× faster. Drag grows with the square of speed, so at a steady swim v = √(thrust ÷ ½ρC_dA) — speed rises with the square root of the push. To go twice as fast costs four times the thrust; the water fights back harder the faster you go.
Open the demonstration →What sets its pace?
“A neural network sets the fish's swimming pace. How big is its brain?”
One hundred and forty-one numbers. It learned to invert the drag law — name a speed, and it sets the tail-beat that reaches it, against a drag it cannot escape. A small learned motor command, drawn in full on the board.
Open the demonstration →One format, a third medium.
The same engine that runs Flight, Balance, Grasp, Driving and the Soft Hand — the same withhold-guess-prove beats, the same editable source, the same rule that nothing is animated by hand. Only the medium changed: from air and ground to water, where the body is held up for free and fought harder the faster it moves. Six machines now, across every way a body meets the world.
Guess first
A real question, refused an answer, until you commit. This one asks what a swimming robot spends to hold its depth — and the answer, nothing, is the surprise the water hands you.
It cannot lie
Every claim is checked live by the open swim core: buoyancy cancelling weight to the newton, speed rising only with the square root of thrust, the 141-parameter brain hitting 24 of 24 target speeds. Verified, not asserted.
Hardware to brain
One fish carries the whole stack: the buoyant body, the quadratic-drag rule that shapes every stroke, and the learned policy that sets its pace — the three layers of an embodied system, a sixth time, in a new medium.