It's very, very hard to describe.
Sometimes it starts like an odd fluttery sensation in your tummy and you stop talking to people around you, as you try and grasp hold of the sensation with your mind. You try and get your brain to assess whether this is just a dodgy stomach type feeling, or could this be it.
Then it's like a snowball that's rolling faster, and faster, and growing, as it rolls around in your stomach, but by the time you have grasped that, there's already no air in the room to breathe and your heart is racing. Your heart starts to pound so hard, it feels like it will break through your chest and you hear a rushing sound in your ears, getting louder and louder. I've never been to Niagra Falls, but I imagine it's like the sound at the bottom where the water crashes down, but it's inside your head.
It feels like you are hanging at the top of a rollercoaster, just about to go over the edge, and the length of time it feels like that can vary, but it's always followed by the feeling that you are falling, even though you know you are standing or sitting on terra firma. Sometimes, there's a feeling that you want to get up and run somewhere, but this competes with the feeling that moving might be worse, so you need to stay still. This often results in a weird flapping of the hands motion, as you try to express the indecision. Often the inability to breathe leads to shaking, and more shaking, and an indescribable feeling of something building like waves and crashing over you, but is not recognisable as a fear you can comprehend. There is a definite feeling that everything is swirling round and round in your head, and in the room, and with this is an urge to lie on the floor because surely if you get down there, the sensation of spiraling and falling will stop.
In those moments I'd like to listen to Corporal Jones
but I'm more likely to believe Private Frazier.
Apologies to those who don't get the references.
In those moments, all you can do is hold on for dear life.