The name has nothing to do with Bougainville. William 'Billy' Mitchell was a U.S. Army brigadier general who spent the 1920s arguing, loudly and often insubordinately, that airpower would decide the next war. He sank captured German battleships with bombs to prove his point, feuded with the Navy, and was eventually court-martialed in 1925 for publicly accusing his superiors of 'almost treasonable administration of the national defense.' He died in 1936 without living to see his ideas vindicated over Pearl Harbor or Midway. By 1944, when Allied pilots flying over central Bougainville named the crater lake beneath them 'Billy Mitchell Lake' - sometimes 'B-25 Lake,' after the bomber Mitchell had championed - his rehabilitation was already underway. Congress would posthumously promote him to major general in 1946, and the U.S. Air Force, which became an independent service in 1947, would claim him as a kind of patron saint. How a volcano on the far side of the world came to carry his name is less documented than you might expect. It may have been simply that Allied mapmakers saw the caldera from a B-25 cockpit and needed a name.

Billy Mitchell has erupted at least twice in the last thousand years - once around 1030 AD, once around 1580 AD give or take twenty years. Both eruptions registered a Volcanic Explosivity Index of 5 or 6, putting them among the largest events of the entire Holocene epoch in Papua New Guinea. The 1580 eruption produced pyroclastic flows that probably carved the modern caldera. The ignimbrite - welded volcanic ash - from that event extends 22 kilometers from the crater to the coast, and its total volume is estimated at around 10 cubic kilometers. That is an extraordinary amount of material. The eruption is believed to have dropped worldwide average temperatures by 0.2 to 0.3 degrees Celsius for a year or two afterward, as sulfate aerosols spread through the stratosphere and dimmed the sun. The link to the mid-1550s signal in Antarctic ice cores remains unsettled - the dating doesn't quite align, and the silica content of the ice-core glass runs across a wider range than Billy Mitchell's typical chemistry suggests - but the volcano is very much still in the conversation.

Today the crater holds a lake roughly three square kilometers in surface area, sitting at 1,013 meters above sea level with a maximum depth of 88.3 meters. The water is cool and clear, ringed by the steep inward walls of the caldera and fed by rainfall. Only one fish species lives in it: Anguilla megastoma, the Pacific long-finned eel, which migrates as a juvenile from saltwater to freshwater, lives for decades in the lake, then returns to sea to spawn. The lake drains into the Tekan River, which eventually reaches the Pacific on Bougainville's east coast. The Rotokas people, whose ancestral territory includes this part of the Emperor Range, have guided visitors to the caldera for decades. The 2015 establishment of Rotokas Ecotourism was an attempt to channel this kind of visitation through community hands rather than through the copper-colored wounds that mining left elsewhere on Bougainville.

Billy Mitchell is not alone. The Emperor Range is a chain of volcanoes stretching across central Bougainville - Mount Balbi to the north, still venting sulphur and steam; Mount Bagana just to the south of Billy Mitchell, one of the most continuously active volcanoes in the entire southwest Pacific, trailing a plume of gas visible from 50 kilometers out at sea. Standing in the Billy Mitchell caldera, you can sometimes see Bagana's plume rising over the intervening ridgelines. The three volcanoes together trace a single tectonic reality: Bougainville sits on a volcanic arc built by the subduction of Pacific Plate material beneath the Australian Plate. The island is still growing, still settling, still occasionally reminding its inhabitants of that fact. A magnitude 6.4 earthquake in 2013 centered just south of Panguna rattled homes across the island. Billy Mitchell has been dormant for nearly 450 years, but dormant is not extinct. If the volcano ever wakes again, the eel in the lake will be the first to know.