"The Greatest Adventure of All" by Ian McHugh

First appeared in Coyote Wild.
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As the story alludes, this traverses Flatliners territory:

"Of course, the rest of us were fear junkies too, otherwise we wouldn’t be sneaking around the labs after hours like the cast of Flatliners."

A cabal of young experimenters try to see what death is like, what lies beyond on the veil. When they get back, some do not buy the results that other cabal members say about the afterlife.  Animals, however, do not return quite as shaken up.  The cabal keeps pushing the boundary.