Alternobaric vertigo is one of those moments that changes how you descend forever. It happens when one ear equalizes and the other lags behind, and the pressure difference between labyrinths sends contradictory signals to your brain. The result is nausea, disorientation, and — worst of all — an overwhelming urge to bolt for the surface at exactly the wrong moment.
It happened at Cabo de Gata, June, water at 19 °C, descending the shotline toward 18 m. Everything was normal — equalizing every metre, same as always. At 14 m I noticed my left ear wasn't keeping up, but I kept going for one more metre, thinking it would sort itself out. Wrong call. When the left ear suddenly gave way at 15 m, my brain received a violent signal from one side while the other was perfectly settled. The world flipped. The shotline looked horizontal. My bubbles were going sideways.
Here's what happened physiologically: the vestibular system uses two labyrinths, one per ear, to feed orientation data to the brain. When both are at the same internal pressure, everything functions normally. When one has a delayed equalization and the other is current, the brain detects a massive asymmetry and interprets it as movement. That's where the vertigo comes from — and the urge to be sick underwater, which is among the least enjoyable situations diving has to offer.
There are three common causes: an ear with an undetected mild cold or congestion; an asymmetric equalization technique, where a diver only equalizes cleanly on one side; and descents that are too fast, without giving both ears time to settle. Most cases are preventable simply by equalizing before you feel discomfort, not after it starts.
The protocol when it happens: grab something that isn't moving — the shotline, the bottom, your buddy — close your eyes for a moment, breathe slowly, and ascend 1 to 2 m until the lagging ear releases pressure. That's usually enough for the vertigo to clear in 30–60 seconds. What matters is not shooting straight to the surface; the sudden pressure change can make things worse and opens the door to other problems.
The worst part of alternobaric vertigo is the sensory conflict. Your eyes tell you that you're vertical in the water. Your vestibular system insists you're spinning like a top. That contradiction is what triggers the nausea — not the pressure itself. Close your eyes, focus on your breathing, and you remove one input from the equation; the brain settles faster.
After the incident, the dive is over — call it at the surface. No continuing the descent even if you feel fine again. The ear is already irritated, and forcing another equalization on the same dive is like pressing on an open wound. It typically leaves the ear sensitive for 24 to 48 hours; no diving during that window. If sensitivity persists beyond that, see an ENT specialist.
One preventive habit that actually works: equalize gently before entering the water so both ears are clear from the start. And equalize every 30–50 cm for the first 6 m, not every metre. The first few metres are where most asymmetries appear because pressure changes fastest in percentage terms at shallow depth. Once you're past 10 m, you can spread out the intervals.
My personal rule after that scare: if an ear takes more than two gentle attempts to equalize, I don't push it. I go up half a metre, try again, and if it still won't clear on the third attempt, I abort the dive. Written out like this it sounds excessive, but when you've lived through alternobaric vertigo you understand that a stubborn ear is reason enough to walk away. The dive site isn't going anywhere — you can come back tomorrow.