I didn't go through the tunnel on it's Day 1 - but I did on Day 2 !! And of course many times since.

The underwater section is 22 miles but as you can imagine there are long approach gradients as the tunnel works down to well under the sea, so the tunnel is another 10 miles in length. Total transit time is invariably 20 minutes nowadays entry to exit, of which about 14 minutes will be under the sea. Cruising speed of passenger trains about 75 mph. If you know Dover the tunnel runs out under the sea right under the town and harbour (though there is no sign of it).

The Channel is surprisingly shallow, to the extent that any ships sunk have to be carefully marked and got out of the way otherwise other ships run the risk of striking the wreck sticking up from the seabed.

Compared to other rail and road tunnels under estuaries the Channel Tunnel is a long way down beneath the sea bed (which they did to get down into the chalk rock which is much easier to dig through).

There is absolutely no sensation of going down or up (there's no flat section at the bottom, it's more of a flattened "W" profile which was done to facilitate drainage engineering). At the one third and two thirds points (say at minutes 7 and 14) there are switches between the two tunnels and if you have an ear for them as the train runs over them you can know your progress.

The transit time is a bit longer now than at first opening because since then the number of other trains has increased and the big, very heavy trains that haul road trucks under The Channel can only manage about 60-70 mph going up the gradients and all the other trains have to fit around them. Sometimes you will notice your passenger train slows up towards the end which is a sign it is being controlled by signals behind one of these "Freight Shuttles", still several miles ahead on the tracks. One quiet evening, in contrast, we cruised effortlessly at 100 mph end-to-end, my fastest transit time ever (I still always clock the time to go through).

On Day 2 I returned from Paris in the dark (it was December) and apart from the announcements most passengers never noticed when we were in the tunnel or out of it. The trains (a variant of the French TGV) are great.

On that inaugural outbound morning trip there was a poor guy seated across the aisle from me, a student I guess, who had also laid out the not inconsiderable sum for the first trip. Obviously an excited train buff he had all his technical books, locomotive histories, maps, timetables etc with him. He also had his girlfriend with him who looked completely bored with the whole thing from first boarding, and just acted sulky the whole trip and seemed to wreck it for him. I'd have left her in Paris to find ther own way home !