## 7 Testing in Hongqi Ice Lake, Changchun, China  
To evaluate the performance of the EFSOS system under cold temperature conditions, an ice tower 15 m in height was constructed on the frozen surface of Hongqi Lake in Changchun City during winter (Figure 11). The ice tower was formed by stacking natural lake ice blocks 60 cm thick. The depth of the lake beneath the ice tower was approximately 3 m, offering a simulated subglacial lake environment. From 22 January to 8 February 2021, drilling and sampling tests were conducted with the ice tower, with the minimum ambient temperature reaching \(-25\)\({}^{\circ}\)C.  
Three downward and two upward drilling operations were carried out, and approximately 660 mL of lake water was collected for each drilling. Figure 12 shows the variation curves of the drilling parameters with depth during the downward drilling of the second borehole. The power of the lower melting tip ranged from 6 to 6.5 kW, with slight variationsowing to cable losses and surface voltage. The lower melting tip temperature (Tlower) was maintained at around 30 \({}^{\circ}\)C during drilling. However, occasional water leakage between the ice blocks in the ice tower led to rapid heat dissipation from the tip surface and a sharp temperature decrease. Subsequently, the temperature of the tip rapidly increased in the dry hole. To prevent the overheating and burning of the cartridge heaters, it was sometimes necessary to reduce the tip's heating power. During drilling, the WOB was generally controlled within a set range of 20-50 kg. The average drilling speed for downward drilling was 2.14 m/h, while the average upward drilling speed reached 3.26 m/h. The inclination of the borehole fluctuated by approximately 1\({}^{\circ}\).  
Throughout the upward drilling process, the temperature of the upper melting tip was consistently controlled and could be automatically maintained below 100 \({}^{\circ}\)C without human intervention. The test data were successfully transmitted via Iridium satellite communication every 10 min. Approximately 1 MB of data were collected per 24 h. The remote-control functionality, generator start-stop, and drilling-mode switching were also tested.  
Another objective of the lake test was to verify the pollution-free sampling capability of the RECAS sonde. Several measures were implemented during the test to ensure a clean drilling process: (1) the exposed parts of the sonde were wiped with alcohol for disinfection;  
Figure 11: Drilling testing in field lake: (**a**) layout of field lake testing; (**b**) drilling downward; (**c**) the lower part of RECAS sonde in the lake; (**d**) drilling upward; (**e**) inside view of control room; (**f**) collecting the water sample from the sampler.  
(2) the sampling cylinder of the sampler was sterilized under high temperature and high pressure; and (3) a UV lamp was positioned near the borehole to provide continuous UV irradiation for sonde sterilization during drilling. Microbial samples extracted from the water samples collected by the RECAS sonde and in situ water samples were sent to a specialized industrial testing institution for analysis. The results revealed that the differences between the two sets of samples were within the allowable range, thus verifying the pollution-free sampling capability of the RECAS sonde.