There are a handful of ultra-high-net-worth pursuits that sound tempting, but don’t really make sense. They’re typically wildly expensive and operationally inconvenient. And always difficult to sustain. The shortlist includes a personal nuclear reactor, a thoroughbred racehorse farm, and a .

Admittedly, though, when it comes to the ultimate superyacht toy, the mini-sub’s appeal is tough to beat. People have been fascinated by what lies below the surface of the ocean for millennia. Mesopotamian ruins from nearly 8,000 years ago include deep-water shells and pearls, and Aristotle wrote about the 11-year-old boy who would become Alexander the Great descending into the sea in a glass diving bell 2,500 years ago.

Even the tragedy of the , which imploded on a dive to the Titanic wreck two year

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