Which of the following is so special in central Nisyros?

Central Nisyros, a jumble of looming caldera ridges and shimmering craters where fumaroles spit hot, sulfurous gases from the earth’s crust, is usually visited only by volcanologists and curious tourists.
   But last Thursday at dusk, 15 musicians gathered at an active volcanic crater for an experiment in improvisational site-specific performance. Lit by a glowing August full moon, they played for 10 hours, asked only to take inspiration from their surroundings.
   Titled "634 Minutes Inside a Volcano" , the event was organized by six d. o. g. s, an Athens arts space, and the Onassis Cultural Center. The musicians, mostly Greek and in their 20s and 30s, came from improvisational backgrounds, including jazz, classical, avant-garde and electronic, and many met for the first time at the sound check.
   The organizers documented the event with photographs and an audio recording, which they plan to release in coming months. No artificial lighting was used inside the crater, so aside from the very beginning and end, when there was sunlight, the performance could not be captured by cellphone cameras.
   Mr. Dagritzikos began coming to Nisyros a few years ago and was entranced by the island’s volcanic terrain, which attracts up to 500 visitors a day in high season, many from Kos, the popular beach-holiday island to the north.
   " It’s completely unique, basically the closest you can get to the center of the earth," Mr. Dagritzikos said. "And if you realize where you are, that around those walls lava erupted 20, 000 years ago and this whole island was created, it’s just mind-blowing. "