They used an autonomous underwater vehicle with upward-looking multibeam sonar (the main one seems to operate at 300-kHz) to map 140 km² as well as measuring ocean currents, temperature, and salinity at 20 to 80 m below the ice. Multibeam was new to me, Wikipedia says this "emits acoustic waves in a fan shape beneath its transceiver [using] beamforming to extract directional information from the returning soundwaves, producing a swathe of depth soundings from a single ping" (https://en.wikipedia.org/wiki/Multibeam_echosounder)

The researchers say that limited observational data on how the sea erodes this ice leads to uncertainty in sea level predictions and this new data should help future studies lower those error margins. I want to say cool stuff (no pun intended) but it's also frightening in a way... at least we'll better know what's coming for those who come after us

I don't see any mention of how close previous assumptions were to what this new information shows; probably this is not yet processed/applied enough to interpret it in that way?

If anyone is interested in the overarching project that produced this paper (I assume; authors are PIs on the project), check out TARSAN:

https://thwaitesglacier.org/projects/tarsan

On the AUV front, as well as multibeam from the Hugin, seafloor imagery has been mapped with an AUV named Rán.

I work alongside the team who build Autosub Long Range, who were there in 2022, taking physical/chemical/biological measurements (CTD, microstructure, turbulence):

https://noc.ac.uk/news/boaty-mcboatface-returns-thwaites-gla...

Underwater Gliders have also been deployed under or near the ice sheet to do similar:

https://www.sciencedirect.com/science/article/pii/S096706451...

The "AUV under ice" thing is a fascinating challenge to me. You have to rely a lot on different and novel navigation techniques (like terrain aided/bathymetric navigation and acoustic beacons). And you have to work out how to get your very very expensive robot not stuck under an ice sheet, when its default failsafe is to become positively buoyant, and its primary method of communication (satellite comms) requires a clear view of the sky :-)

The results of the work are.... sobering? Worrying? But I'm glad we're doing them.