SCOTT DETROW, HOST:
Two years ago, scientists surveyed the floor of Lake Michigan looking for shipwrecks, and they found something mysterious and unexpected instead - a cluster of sinkholes on the lake bed. The sinkholes sit roughly 450 feet below the surface and range from 300 to 600 feet across. Scientists have now photographed them to get a close look for the first time. According to lead NOAA researcher Steve Ruberg, they still have a lot of questions, but they also have some clues based on similar sinkholes in neighboring Lake Huron. The first thing I needed when I spoke to Ruberg was a little bit of a geology orientation.
STEVE RUBERG: The way these systems could have been formed, since we're sitting in the Great Lakes on a - at least the southern Great Lakes, we're sitting on this limestone bedrock. And just like any cave system, as water permeates through these limestone systems, then it begins to erode. And caves form and caverns form, and when a cavern erodes all the way to the surface, it collapses and forms a sinkhole. The ones in Lake Huron, that is exactly what we're seeing. We really haven't gotten to the place for the - we haven't gotten enough data yet for the ones in Lake Michigan to really determine what they are.
DETROW: So the Huron ones at least - and you're looking more in Michigan - despite the fact that it's on a lake bed of this massive (ph) system of water, it's water coming up from below that's creating them?
RUBERG: Exactly, yeah.
DETROW: Interesting. You said you're not quite ready to make that determination in Lake Michigan. What are the questions that haven't been answered yet that you're hoping to learn more about with these photographs, with the next steps here?
RUBERG: Well, what we'll do is go out to some of the deeper ones and see if we see groundwater coming into those. We'll do some mapping. Some of the ones in Lake Michigan do look like limestone karst features that we've seen on land in the state of Michigan. And we'll put the data together. And we'll sit down with a team of experts, and we'll come up with some conclusions.
DETROW: Can you tell me more about the creatures that have been observed living in these sinkholes?
RUBERG: Yeah. Well, the sinkholes that are in Lake Huron, we've seen cyanobacteria. So this is a bacteria that photosynthesizes. And then right in there with them are these chemosynthetic bacteria, and there's a symbiotic relationship between the two of them. And really, it creates these beautiful purple mats that kind of come and go. You know, these little critters will sort of rise up. You know, in the daytime, the photosynthesizing ones get up to get sunlight, and at nighttime, the chemosynthetic ones will go up to get the sulfur that they need.
And then once you get out deeper, then you have just the chemosynthetic bacteria that are operating, because light can't penetrate, so you don't have the photosynthesizing ones. So it's a very different world than what we see in the rest of any of the Great Lakes. The Great Lakes are, you know, this - these beautiful, large freshwater systems, and here you have this sort of salty environment right there in these giant freshwater lakes.
DETROW: What comes next? I know that there's been attempts to pass legislation to fund more exploration of stuff like this in the Great Lakes. What comes next for you and your work?
RUBERG: Well, we hope that legislation gets passed. Only 15% of the bottom of Lake Michigan has really been mapped, and that's comparable to our nation's coasts. So there's really a need to map these systems out. That is one outcome of this. The other outcome of it is just understanding the basic biology that is represented in these systems.
DETROW: And I do want to ask this at the end - we started with people trying to look for shipwrecks. Curious, do you ever come across them in your research?
RUBERG: We actually did come across one as we were looking around in these sinkhole systems. There was one nearby, and we got to look at one. It's a - it was just another part of the puzzle.
DETROW: That's NOAA researcher Steve Ruberg. Steve, thanks so much.
RUBERG: You are very welcome. It's been good talking to you, Scott.
DETROW: You too.
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