After almost 600 comments in two days (blimey, thanks) it'd be remiss of me to move on without reflecting on your responses. Here's my analysis of your highs and lows.
What's the highest you've been this year?
• The easy winner, twice as high above sea level as anyone else, is Will from "travel-unrestricted Switzerland". He's been 2630m up Greppon Blanc near Siviez, as well as over the border into France on his touring skis.
• Second place goes to Friar Sven at 1300m on the third floor of an office building in Lusaka, Zambia, and third to Wayne at 991m outside Whistler, British Columbia.
• My highest UK reader is Putters at 526m, crossing the Buttertubs Pass (in the Yorkshire Dales) to get milk in his Toyota Auris.
• Only a quarter of you have been above 200m this year, but two-thirds have topped 100m.
• I've been no higher than 42m this year, and 87% of you can beat that.
• My most vertically challenged reader is Jochem in the Netherlands, the top floor of whose home scrapes 8m above sea level.
What's the highest you've ever been above ground level?
• Plane flights were excluded, so this was the "tallest building" question.
• Three of you have been to the world's highest observation deck at the top of the Shanghai Tower, 560m up.
• Three of you have been up the Burj Khalifa in Dubai, which is just four metres lower.
• Only 14% of you have been higher than me, all in Shanghai, Dubai or Tokyo.
• My highest ascent was 447m in the SkyPod at the top of the CN Tower in Toronto. 20 of you joined me, making the CN Tower the most common highest point.
• The second most common highest point was the (former) World Trade Center in New York, around 415m, and the third most common was the 3ème étage of the Eiffel Tower (276m).
• A quarter of you, or at least a quarter of those who responded, have never been higher than 300m off the ground. Three of you have never topped 200m, and one's never been higher than 80m at the 120 Fenchurch Street roof garden.
• It'd be fascinating to ask the wider population the same question, I suspect we're more ascendant than the norm.
What's the highest you've ever been above sea level?
• This was the "highest mountain" question. And blimey, you put my efforts to shame.
• Our winner was HTFB who's climbed Stok Kangri, a 6153m peak in Kashmir. That's the equivalent of two-thirds of the way up Everest.
• 8% of you have climbed in the Himalayas, notably in Nepal and Tibet. For some the highest point was a pass, for others a base camp and for others a peak.
• The Himalayas account for half of you who've been above 5000m. The others achieved that elevation in the Andes, Mexico and Tanzania (including 5895m on the rim of Kilimanjaro).
• A quarter of you have been above 4000m, 60% above 3000m and 80% above 2000m.
• For one third of you, your highest point is in the Alps. This includes the most popular summit of all, Aiguille du Midi(3842m) near Mont Blanc, at the top of Europe's loftiest cablecar run.
• After the Alps, the Rockies were the next most popular mountain range. Your ascents here varied from Pikes Peak at 4302m to wandering around Denver at 1609m.
• Ten of you topped out in the Canaries, notably up Mount Teide in Tenerife (3715m).
• My 1105m in Vermont was waaaaay down the list, only beating the nine of you whose highest points were in Britain. Peculiarly none of you had Ben Nevis as your lifetime peak. I so need to up my game.
What's the lowest you've been this year?
• Our winner is Alan in Hong Kong who's been 30m below sea level in the Cross-Harbour Tunnel.
• Just ten of you claim to have been below sea level this year, including two on the tube. Just one of you has been on the Jubilee line through Waterloo (26m), which would be an astonishing statistic in any year other than 2021.
• Three of you have ventured under the Thames in east London, including the Greenwich Foot Tunnel, the Blackwall Tunnel and the Thames Tunnel. In a normal year such journeys would be commonplace and entirely unremarkable.
• Only three of you have been to the seaside in the last four weeks.
• Meanwhile only two of you claim to have been no lower than 100m this year (one of whom lives in a land-locked African country). But I must remember that my question most likely appealed to the lowest of you and attracted a self-selecting sample, so these results are unlikely to be reflective of my readership at large.
What's the lowest you've ever been below ground level?
• This essentially divided you into 'mines' versus 'tunnels'.
• The lowest of the low was a mine. Martin's been 3600m down to the bottom of Western Deep Levels No.3 shaft in South Africa. That's more than two miles below the surface of the earth!
• But the next 25 of you had all been through tunnels.
• The real biggies are all tunnels through the Alps. Seven of you have taken a train through the Gotthard Base Tunnel(2450m), nine through the Mont Blanc Tunnel and eight through the Simplon Tunnel(2150m). The Gotthard Base Tunnel is definitely deepest, so we don't understand why Wikipedia gives the depth of the Mont Blanc Tunnel as 2480m.
• Mines make a reappearance at 400-1000m, with DavidH going deepest (998m) because he used to work down Manton Colliery.
• England's deepest railway tunnel is the Cowburn Tunnel west of Edale at 277m. For three of us, that's the lowest we've been.
• The most popular mine on the list was the Wieliczka Salt Mine near Krakow at 135m.
• Nobody claimed that the Underground is the furthest underground they've ever been.
What's the lowest you've ever been below sea level?
• Huw is our deepest-ever reader, an honour earned 1100m below the coast of northeast England. He's taken the lift down to the Boulby Underground Laboratory at Boulby Mine, located between Saltburn and Whitby, a working potash, polyhalite and rock-salt mine operated by ICL.
• Our next two contenders also ventured down mines. Man of Kent went on a school trip to Tilmanstone Colliery in Kent (600m??) and Estepnist descended to Level 19 of Geevor Tin Mine in Cornwall (480m).
• A quarter of you have been to the Dead Sea, which at 430m below sea level wipes the floor with all rail and road tunnels.
• Robin's been through the current world record breaker, the Ryfylke Tunnel. This road tunnel passes 292m beneath a Norwegian fjord and opened at the end of December 2019.
• Just one of you has taken the Shinkansen from Tokyo through the Seikan Tunnel(240m).
• Let's round things off with the most popular answer of all - as many as half of you said the Channel Tunnel was the furthest below sea level you'd ever been (115m). Thank goodness they built it, because the Jubilee line really doesn't compare.