What is Epsom salt, and should you be bathing in it?

Epsom salt was first discovered in the 17th century in England, and for hundreds of years it's been dissolved into baths to ease aches and pains. In other words, it's nothing new. But over the past couple of years, Epsom salt baths have enjoyed a resurgence, with high-end spas offering float sessions inside sensory deprivation tanks and celebs like Gwyneth Paltrow and Hannah Bronfman revealing they swear by their therapeutic effects.