Drones, geology, and the tropical sun: a case study

On the slopes of Maunakea, a short distance east of the Visitor Information Station and Hale Pōhaku, a small pit crater (sometimes evocatively called “Bottomless Pit”) can be seen on satellite imagery.

I've circled it in this wider view of the area, it's not very big.

I thought for sure I'd mentioned it before on this blog, but a search failed to turn up anything. I find myself surprised at this, because I've been aware of this crater for over a decade now, and I've wondered how deep it goes for nearly as long.

You see, this crater is a bit unusual. There are many puʻu, or cinder cones, on Maunakea, and plenty of eruptive vents on Mauna Loa and Kīlauea. But from the ones I've seen up close, such eruptive vents are always filled in pretty close to the surface with hardened lava or choked with tumbled stone. This pit crater is unusual because, alone of all the craters I've seen on this island in Google Maps (and I've spent many hours looking), it has a dark hole at its center.

A closer view of the crater, with the hole clearly visible.

My first encounter with this crater actually came all the way back in 2012, which is how I know I must've discovered it within a few years of moving to the island. I don't remember the exact date, other than that it was on (or very close to) the night of a full Moon, and (for a few reasons) probably sometime during the summer.

I was working at the Visitor Information Station in 2012, and sometimes I'd have multi-day shifts, where I'd stay overnight in one of the rooms at Hale Pōhaku. I think I must've recently discovered the crater at the time, as I was very interested in hiking out to see it. I didn't have a car at the time, so while I could ride up to the VIS for work, I didn't have time during the day in which to visit the crater. (This recent discovery/fixation may help explain what follows.)

I'd just finished a day of work and gotten back to my room at around 11 PM, when I was struck with a crazy idea: why not hike the roughly half-mile distance out to see the crater? It was a full Moon, and there was plenty of light. (There were some light clouds steadily blowing west overhead, but not enough to seriously dim the moonlight.)  To my early 20s brain this made perfect sense; I was in pretty good shape, and could still miss an hour of sleep without feeling it too harshly the next day, so without telling anyone I was going for a nocturnal jaunt I grabbed my trusty headlamp and set off to the east.

In retrospect of course, going for a high-altitude hike at night to a geologic feature of unknown depth without telling anyone has Not A Good Idea written all over it. As mentioned, I think this must've been during the summer, as I remember the night being relatively mild, but it still wouldn't have been very fun to have been caught out overnight if I'd, say, sprained an ankle. And one thing I didn't know before setting out is that the ground between Hale Pōhaku and the crater is some of the best terrain I've ever seen for twisting an ankle, being mostly loose soil with fist-sized rocks that turn over at the touch of a feather (or at least a hiking boot).

Still, after half an hour or so of trudging over hill and dale (there's a ridge between Hale Pōhaku and the crater which isn't apparent on the map), I made it to my target, approaching from uphill (the northern side). As I crested the rim of the crater, I had an impression of it lying sprawled before and below me, and in its center, a yawning maw of inky blackness large enough to swallow a person. The full Moon overhead, the patchy clouds, the just-completed trek through silent shadowy solitary surroundings, and the anticipation I had for the event all combined to engender a feeling of solemn awe and make it a momentous, even portentous event.

I felt this feeling of awe for perhaps a full second, maybe even two, when a bird suddenly exploded out from where it was nesting not far from where I stood in a loud whirring of wings and squawk of alarm, giving me the fright of my life and nearly startling me over the edge of the crater as I reflexively jerked around.

I managed to retain my balance and not fall into the crater (thankfully, as will become apparent). I think, after recovering from the fright that hapless avian denizen gave me, I tried to take a few photos, but the light wasn't enough for them to come out. To make a long story short, after some more minutes admiring the awe-inspiring nighttime scene I headed back to bed with no one the wiser about my ill-advised midnight perambulation. And that was actually the last time I visited the crater for a full decade.

But even apart from it, all those years, I continued to wonder: just how deep did that pit go, which had left such an indelible impression upon my younger self? In those intervening years, I moved to Australia, got a PhD, and moved back to the island, and in between something new entered the pictures: drones.

While consumer-facing drones have been around since well before I moved to Australia in 2017, early models were cruder and less functional than the ones available today, and I wasn't really aware of their slowly improving capabilities. My vague impression was that they were mildly interesting toys, but not much more. (I also didn't exactly have the disposable income to buy one at the time, so I wasn't paying much attention.) Regardless of how accurate my assessment was at the time, it's true that drones have only continued to improve since, and in mid-2020, while stuck at home in lockdown in Melbourne, I started coming across videos on YouTube that opened my eyes to just what modern drones could do.

Enter another piece of information: Hawaii's location in the tropics. This fact means that, twice a year as the Earth progresses along its orbit, the Sun passes directly overhead. I don't remember when I first put two and two together, but at some point I had the thought that, at just the right time, the Sun would be directly overhead, casting its rays down the mouth of the pit…and that if I were to, say, be flying a drone over it at that time I could get an idea of just how deep the rabbit hole pit crater went.

Lāhaina noon (the modern name for when the Sun passes overhead) happens in May and July for Hawaiʻi island. (It happened last week, July 24th, in Hilo.) Last year I was excited to get my Mini 3 Pro in June, in time to catch the Sun in July. I even hiked out to the crater for the first time in a decade the week before Lāhaina noon to scope the area out, only to fall sick that week and be unable to head up Maunakea on the day itself. Still, I wasn't discouraged, and philosophically reasoned that a year's experience with drone flying would help me next time around. This year the May Lāhaina noon happened in the middle of the week, but the July one happened over the weekend of the 22nd/23rd, so I possessed my soul in patience and bided my time. I did get some pictures, though:

The crater July 2022, with the pit in the middle. A bit less spooky under the summer Sun.

This past Saturday, July 22nd, my chance had finally come: I headed up Maunakea with my Mini 3 Pro (and a friend), hiked out along a trail downhill from the crater, and as the Sun reached the zenith, I sent my drone skimming through the air towards the pit.

The crater July 2023, from the air. The hole's a bit bigger than a person, for scale.

And after a decade of wondering about this particularly intriguing geologic formation, I finally had my first glimpse at an answer regarding its depth! Everything, to my surprise, worked out perfectly: the Sun blazed away overhead, its light illuminating the pit, and to my surprise (and delight), it turned out to be significantly deeper than I'd been expecting.

Behold, the pit crater's floor, clearly visible.

A decade of wondering, years of imagining how this moment would go, and I can still hardly believe it went off without a hitch: I was able to clearly see and photograph the floor of the pit. A still photo doesn't show the parallax I saw while flying over it, but from that I would estimate that the bottom of that pit has to be on the order of 10 meters (~30 feet) down, or perhaps more. I took some videos of the effect, and I hope to make a slightly more rigorous estimate before too long, but I wanted to share this photo soon after fulfilling a multi-year dream of mine. (Yes, some people's dreams are inspiring stories of overcoming hardship or taking years to achieve something significant; mine pretty much boils down to, “I wonder how deep this hole goes?”)

It also deepens (pun not intended) the mystery of the hole's origin. There are plenty of cinder cones on Maunakea, especially around this part of the mountain, which are created from explosive eruptive events (which pulverize rock into cinder), but this isn't a typical cinder cone. Looking at satellite photos, the area nearby seems to show signs of a lava flow, and I've read that some of the last eruptions on Maunakea happened around this general area. Perhaps this particular vent was erupting more fluid lava rather than explosive cinders (which tend to fall back and fill in the vent)? That might explain the strangely smooth-looking floor (rather than a jumble of rocks as might be expected): maybe magma, after erupting, drained back in and we're seeing a frozen lava lake from the last lava to emerge from this vent? The area in the crater seems a bit sharp and jagged to have been erupting pāhoehoe, so perhaps it was ʻaʻā, or perhaps I'm barking up the wrong tree entirely. I'm not a geologist and I have no firm explanation for this unusual feature, I just know that of the many eruptive vents and cones I've seen around the island I don't know of another one like it.

A question answered, more questions raised; so it goes in science, and perhaps it's fitting that the pit retains some of its mystery even after being illuminated by the tropical sun. I've got some thoughts on how I might better estimate the depth of that floor, so look forward to seeing something about that when I find time and motivation to work it out some more. In the meantime, this is getting long so I'll leave it here. A hui hou!