Uncoiling wires.
Tom, Same problem and solution as for anti-tank missiles.
You don't mount the spool on the surface and try to drag it downwards, you put the spool on the probe, and it unreels wire that freezes as it goes down. Your total drill/melt depth is constrained by total wire on the spool.
(While on this subjest, anyone else notice that the A/A missiles used in Kenya were actively quided wire-controlled, not IR sensing. Wrong missile for the job, methinks, but don't tell anyone)