Mt Everest at 29,029 feet high (5.4979166 miles) is the highest point. Mariana Trench at 35,814 feet (6.7529545 miles) the DEEPEST. SO?

I get claustrophobic so the ocean deep would not be for me. But I am not athletic nor robust enough or young enough to attempt a climb up to the top. HOWEVER ...a helicopter ride up and landing on top would suit me to a T! Cheating of course but I would not hide it. I would not pretend I'd made the climb. I think looking out on the world from the top of Mt. Everest would simply take my breath away! Thank you for your thoughtful analysis Durdle. I can't imagine anyone not getting loopy almost 7 miles down. And then how long would you have to take to get back up? Oh I guess you don't get the bends in a Bathysphere do you? I'll ask. :)

Well, that' neither you nor I reaching the summit of Everest then.

I  certainly could not climb it, and helicopters can't fly high enough! Even so it must a wonderful experience, as you say.

The craft they use for exploring the oceans is a bit more sophisticated than a bathysphere, but they certainly would not suit anyone with claustrophobia. Bends is not a problem though as the air inside them is at normal atmospheric pressure.  I think they do take a long time on both the descent and ascent so the structure takes the pressure changes gradually, but I don't know how long. The craft is fitted with a depth-gauge but I don't think you'd have any sensation of depth once you are out of daylight.

It imagine it's bit like being a cave where you know your are underground but there's no sensation of the thickness of rock above the passage roof - though you are aware of having gone in for a long distance, or downwards to a considerable depth.

A lot of oceanic work is now done with un-manned, remotely-controlled submersibles with floodlights and cameras. The operators stay in the survey-ship on the surface.

I expect it's very dark the deeper you go so whatever contraption they use must have flood lights to see what is down there in the deep blue sea. Going down into a cave would be out of the question for me but I wonder how I'd do in a submarine...one like Captain Nemo had with all the luxuries? I wonder will we ever KNOW every species that exists or is that beyond our reach? We are told of the tons of species that are extinct. HOW DO THEY KNOW? Mystery upon mystery enveloped in mystique or magic? Beats me Durdle! Thank you for your thoughtful reply and Happy Tuesday to thee and thine. Doing okay still viruswise hopefully?

Sunlight can only reach a few hundred feet down into the ocean, and below that is completely dark. The water is not as clear as you might think, either, but contains a lot of fine sediment - a lot of it is fish poo! - slowly setting to the bottom. That helps to scatter the light from the flood-lamps, reducing their range.

From what little I know about them, the only submarines that give you a first-hand knowledge of being underwater are the small research ones with viewing-ports. I think there are some tourist versions too, that take you on trips in fairly shallow waters close inshore or along coral reefs. The nearest I've been to anything like that was a glass-bottomed boat operating near my home. Naval submarines are not noted for luxury, but the modern ones have comfortable mess-rooms that give the crew a bit of off-duty respite from working inside a big steel tube crammed with machinery and electronics. 

I read once that because there is nowhere inside one of those giving a field of view more than maybe a hundred feet at most, crew-members on long-duration patrols tend to lose some sense of distance. So on coming ashore they are advised not to drive for a few days, to become accustomed to judging distances again.

The extinct species that are known, have left traces like fossils or bones, or are ones that have died out within human history so were recorded. Most of those recent ones were wiped out by humans, like the dodo. New species still come to light though, as fossils not previously found. Occasionally, thanks to better techniques and accumulating knowledge, fossils are re-identified as "new", not the genus or species originally thought.

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Virus-wise... Still OK thank you. I have my first vaccination tomorrow! Happy Tuesday to you too. And it even is Tuesday - I don't  come on AnswerMug everyday day.

I noticed Durdle. I figure you have other fish to fry. What is the longest amount of time any crew stayed down? Years? Do nuclear submarines have that capability? Lurking lurking lurking underwater for years doesn't appeal to me. On the other hand I've never tried it so it might not be all that awful. Thank you for your reply! :) Do you know which vaccine it is?