The original sensors were Geotech Instruments KS-54000 and Guralp CMG-3Tb instruments. The space around the instrument at the bottom was filled with sand so as to minimize temperature changes from air currents in the hole. These holes are dry, unlike IceCube, and they close very slowly due to the movement of the ice sheet. So far it has been possible to remove the instruments and replace them with newer units.
There actually are four instrumented holes--three at full size and depth, and a fourth 6" diameter hole at 500' where one of the drill heads was lost. That hole is currently (2008) being used to test a prototype "cold" sensor that is smaller in diameter because it does not require insulation and heat trace. That sensor is to be replaced with a newer prototype in 2008-09...and the project hopes to install these newer units elsewhere in Antarctica and in Greenland.
At left is something a bit more recent than the rest of the photos on this page...although it also depicts relevant history. This is a January 2017 photo of the signature wall inside of SPRESSO, as documented by 2017 wo research associate Adam West. For more recent info about the place, here's his blog post.
Note that I've seriously overenhanced the photo at left in an effort to make the names more legible...although I must offer apologies to the folks who didn't have a Sharpie with them. And I must also note that that full size photo is 742kb, a bit larger size than I normally use, in deference to Polies and others with slow internet. Adam's original unmodified photo is here...5.2mb. Yes, I recognize many of the names. No, my name isn't there; I never visited. For reference, here's the floor plan:
Note that to the right of the signature wall are the hinges to the freezer door, per the floor plan.
Generally these photos were found on the common drive and were provided courtesy of Kent Anderson, who took them unless otherwise indicated: Don Anderson (DA) and Terry Gacke (TG). Other project members included Jared Vineyard and Rhett Butler. Other construction photos are from Jack Corbin (JC), and a few more recent photos are by Neal Scheibe (NS). Some of these also appear on the USGS web site and/or the IRIS image gallery. The project is funded by NSF, the Incorporated Research Institutions for Seismology (IRIS), and the USGS...also see the USGS/IRIS data sheet about the project and this 29 December 2002 Antarctic Sun article. Oh, and special thanks to Kent Anderson!
While all this was happening...the Dark Sector Laboratory (DSL) was getting completed (mostly) in the dark sector...