In 2257 Discovery entered the mycelial network to rescue Sylvia Tilly and found Hugh Culber alive there, restored by the network after his death the year before.
A being from the network had taken the form of Tilly's late friend May Ahearn and pulled Tilly into the mycelial realm. Discovery partly entered the network to find her, and Michael Burnham and Paul Stamets went into it to look for her.
They discovered Culber, whose consciousness had survived and been given a new body. Stamets and the crew brought him back to Discovery along with Tilly, though Culber struggled afterward to come to terms with his return.