How Spectroscopy Can Explore the Underworld on Earth

Not coming from a spectroscopy background, my five-plus years as a member of the Spectroscopy editorial staff have required me to have an inquisitive mind to learn about the latest techniques and innovations in analytical spectroscopy. My lack of knowledge in this space required me to not only ask lead investigators and industry experts about how these techniques and analytical technologies worked, but also how using them in certain application areas was important. Essentially, I had to ask researchers: Why does your research matter?

To that end, as a spectroscopist outsider, I always sought to try and seek out stories that would interest not just industry professionals and spectroscopists, but the general public as well. Recently, I stumbled upon a book written by Susan Casey titled The Underworld: Journeys to the Depths of the Ocean, and because I enjoy learning more about new topics, I opted to read it (1). In her book, Casey takes readers on an expedition into the relatively unexplored frontier known as the deep sea, where darkness, pressure, and mystery are the three guiding principles (1). In many ways, the deep sea contains more unknowns than the vast reaches of outer space.

Deep sea blue illustration background realism. Dark deep and scary ocean banner. | Image Credit: © Konstantin Gerasimov - stock.adobe.com

In the book, Casey balances between highlighting the weirdness of deep-sea life and how they’ve adapted to life in the abyssal and hadal zones (areas in the deep sea where no sunlight reaches), while giving attention to how advancements in technology have enabled explorers to advance the field (1). Casey describes organisms that produce their own light to attract prey or mates and the extreme sexual dimorphism and parasitic mating strategies of some deep-sea fish (1). She explains the physical reality of depth, and how pressure rises steadily with every meter of descent and how cold, darkness, and sparse food shape evolutionary solutions that seem almost alien to surface dwellers (1). It is also because of pressure that has prevented much exploration of the deep sea, because the technology simply does not exist to venture that far beneath Earth’s surface.

Much of her book focuses on the expeditions undertaken by the famous Alvin submersible and remotely operated vehicles (1,2). She profiles how advances in this deep-sea technology have influenced how much scientists could observe in the deep sea. And did you know that spectroscopy has played a big role in conjunction with the Alvin submersible?

Underwater Robot Exploring the Ocean Depths. Advances in technology have allowed researchers to explore more of the deep sea. Generated with AI. | Image Credit: © hongjian - stock.adobe.com

It didn’t always used to be this way. Alvin was first launched in 1964, and because of technological limitations at the time, spectroscopic instruments were not attached to this submersible (2). However, as technology has advanced, so has the capabilities of what Alvin was able to do.

Since the 2000s, Alvin has been upgraded with spectroscopic sensors and instruments either directly onboard or deployed through remotely operated tools that it carries (2). These tools have performed various functions that have allowed Alvin to continue the expedition into the deep-sea environment.

There are four main tasks that spectroscopy has been used in Alvin’s operations. The first is in situ chemical analysis (2). Alvin has been used to deploy Raman spectrometers to analyze minerals, fluids, and gas hydrates on the seafloor. Raman spectroscopy has allowed researchers and oceanographers to identify molecular structures and chemical compositions in situ. For example, researchers determined the presence of methane hydrates, carbonates, sulfides, and other compounds around hydrothermal vents (3). The Deep Ocean Raman In Situ Spectrometer (DORISS) and its successor DORISS II were specifically designed for use with Alvin and other submersibles for this purpose (4,5). DORISS and DORISS II comprised three components: the spectrometer, the laser-power supply telemetry unit, and an optical head (4).

Second, spectroscopy has enabled scientists to measure the chemical gradients in hydrothermal vent fluids, such as sulfide, methane, and carbonate species, without harming or damaging the sample (6,7). These data helps explain how chemosynthetic life thrives in these extreme environments and how geochemical energy drives deep-sea ecosystems in the absence of sunlight.

Third, newly developed laser and optical spectroscopic systems have been used for gas and fluid identification (5). These systems have been integrated with Alvin’s sampling arms, and it has helped distinguish between methane (CH₄), hydrogen sulfide (H₂S), and carbon dioxide (CO₂) gases trapped in vent structures (2).

And finally, spectroscopy has been helpful in studying ocean life and the biological systems that exist down in the abyssal and hadal zones. It has been used to study pigments and compounds from deep-sea organisms collected or observed by Alvin, including the detection of fluorescent proteins and bioluminescent emissions (8,9). This is important because these observations, made possible with spectroscopy, have allowed scientists to learn more about biochemical adaptations that have occurred because of the deep-sea’s dark, high-pressure environment.

Although Alvin itself is primarily a manned research vehicle, it has been instrumental in deploying and operating spectroscopic technologies in the deep ocean. The integration of tools like DORISS have helped advance oceanography even further. Tools like DORISS have allowed scientists to perform non-destructive, real-time molecular identification of seafloor materials. Because of these advancements, Alvin has evolved into something more than a sampling platform; it now functions as a mobile underwater laboratory capable of advanced spectroscopic analysis.

As a result, once I finished The Underworld, I learned that the book not only serves as a medium to inspire and fascinate readers. The book also promotes the science of spectroscopy indirectly. Through the documentation of Alvin’s voyages, Casey demonstrates that it is these technological advancements that have helped propel deep-sea exploration and oceanography further.

References

1. Casey, S. The Underworld: Journeys to the Depths of the Ocean. Knopf Doubleday Publishing Group; New York; 2024.
2. Woods Hole Oceanographic Institution, HOV Alvin. WHOI.edu. Available at: https://www.whoi.edu/what-we-do/explore/underwater-vehicles/hov-alvin/ (accessed 2025-10-27).
3. Breier, J. A.; White, S. N.; German, C. R. Mineral–microbe Interactions in Deep-sea Hydrothermal Systems: A Challenge for Raman Spectroscopy. Phil. Trans. R. Soc. A. 2010, 368, 3067–3086. DOI: 10.1098/rsta.2010.0024
4. Brewer, P. G.; Malby, G.; Pasteris, J. D.; et al. Development of a Laser Raman Spectrometer for Deep-ocean Science. Deep Sea Res. Part I: Ocean. Res. Papers 2004, 51 (5), 739–753. DOI: 10.1016/j.dsr.2003.11.005
5. Monterey Bay Aquarium Research Institute, Laser Raman Spectroscopy. MBARI.org. Available at: https://www.mbari.org/technology/laser-raman-spectroscopy/#:~:text=We%20have%20extensively%20adapted%20and,separated%20from%20the%20oil%20sample. (accessed 2025-10-27).
6. Nuzzio, D. B.; Taillefert, M.; Cary, S. C.; et al. In Situ Voltammetry at Deep-Sea Hydrothermal Vents. In Environmental Electrochemistry; American Chemical Society: Washington, DC, 2002; Vol. 811, pp. 40–51. DOI: 10.1021/bk-2002-0811.ch003.
7. Ding, K.; Zhang, Z.; Seyfried, W. E.; Bradley, A. M. Integrated In-situ Chemical Sensor System for Submersible Deployment at Deep-sea Hydrothermal Vents. Oceans 2006, 1–6. DOI: 10.1109/OCEANS.2006.306842
8. UC San Diego, UCSD – Scripps Institution of Oceanography. Available at: https://scripps.ucsd.edu/news/scripps-researchers-explore-deep-sea-methane-seeps-alvin-submersible (accessed 2025-10-27).
9. Woods Hole Oceanographic Institution, History of Alvin. WHOI.edu. Available at: https://www.whoi.edu/what-we-do/explore/underwater-vehicles/hov-alvin/history-of-alvin/ (accessed 2025-10-27).