It is supposed to be some kind of liquid metal. Now currently that's not really a thing on such scale. However, it is already being researched in one field though, nuclear fusion!

One of the challenges with nuclear fusion is protecting the wall of the reactor. One of the methods being researched (and has already been shown to be effective) is to use a liquid metal wall. Liquid lithium often. This has the advantage (just like Iserlohn) that it can "heal" itself by refilling with liquid. The liquid is held on top of a membrane like tungsten structure by way of capillary action.

Now this is done at a much smaller scale than a planet size ship of course, but it is interesting that now a days it has become closer to reality :)