There are several places in the park where this phenomenon occurs, from Cades Cove all the way to the Elkmont Campground.
This was a blending of two shots: one of the location itself at around dusk to ensure the area was in focus (and at a lower ISO); and a 27 minute exposure once sundown had occurred. I would recommend scouting out locations earlier in the day in order to find interesting backgrounds, and use minimal lighting headgear to make any adjustments to your setup right before the flies emerge as it can disorient them for a bit. Any light intrusion can ruin your image! Also, make sure to turn on the Long Exposure Noise Reduction and to have a fully-charged battery, as this will take you at least until midnight to accomplish the first shot. Gear: Sony A7RIII, 16-35 f 2.8 G-Master lens at f2.8, 30 mm, at ISO 1600, for 27 minutes.
Photographing the synchronous fireflies in Great Smoky National Park had long been a bucket list item for me, and with the growing popularity of the famed Firefly Lottery at the beginning of June, it was beginning to look like getting into this particular section of the park was going to be a challenge. However, I took a chance and just went a few weeks earlier in the late spring of 2019 and found the area all to myself. To witness this marvelous phenomenon of nature in the quiet stillness of the woods was indescribable, and one that I am intending to participate in again in 2020.