The party I DM a Princes of the Apocalypse campaign for has recently gotten a second Bag of Holding, and they asked what would happen if they put one in the other. With a successful Arcana check, they learned the description from the DMG (p.153-154) with an emphasis on the following:
Placing a bag of holding inside an extradimensional space created by a Handy Haversack, Portable Hole, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it to a random location on the Astral Plane. The gate then closes. The gate is one-way only and can't be reopened.
They immediately saw potential in this as what one player referred to as a "nuclear option" if things got really hairy. Being PotA, this could potentially happen in the presence of an elemental, like Imix or Olhydra. As I tried to prep for this possibility, I read Monte Cook's A Guide to the Astral Plane. On page 20, it is noted that:
More than any other plane, the Astral is cut off from the Inner Planes. The dense, unadulterated intensity of the elements there is the antithesis of the Astral Plane. No elemental can exist on the Astral...
So, if the Bag is placed in the other Bag and the elemental is within 10 feet, will the elemental simply not transported or is it destroyed?