Lava lakes are exceedingly rare. Despite our hair-raising journey to get there, Marum is considered one of the more accessible. Two larger lava lakes (Erta Ale and Nyiragongo) are in remote, conflict-ridden corners of Africa, and another is all the way down at Mt. Erebus, in Antarctica.
Mackley has been coming here for 14 years because there is one thing that eludes him: He wants to get to the very bottom and be right next to the lava.
"That's where I'm going to go," Mackley told us. "To get a shot of a human being looking like an ant on the lip of it, that will show the awesome power of Mother Nature. That's really the ultimate shot."
He came tantalizingly close last year. A few years earlier it all nearly ended in disaster. His campsite was completely destroyed by stormy weather, and his entire team had to be rescued. This time, however, he has come prepared, spending thousands of dollars hauling state-of-the-art gear up to the volcano. To get down to the lava, Mackley has also brought a team of like-minded daredevils -- professional climbers and cameramen -- and a whole lot of rope.
As we tried to take another peek at the lava, the wind suddenly picked up, and us almost along with it. It roared past as though it were being sucked into the crater. Below us the volcanic smog quickly moved in, and the lava lake disappeared.
It was as though the volcano was telling us that we had seen enough.
"Marum is a fickle b----," Mackley scoffed, looking more determined than ever. But it was clear Mt. Marum wasn't going to go down without a fight.
Watch the full story -- and find out if Geoff Mackley's team made it down to the lava -- on "Nightline" tonight at 11:35 ET.
ADDENDUM: Our helicopter pilot, Phil Cotter, died Aug 26, 2011, when his helicopter crashed on an outlying island in Vanuatu. Our condolences go out to his family and friends. It was a pleasure flying with him.