We don't know just how thick it is, probably miles thick, so it's very expensive and very difficult to go down there -- send down your submarine or whatever it is -- and explore.
厚さは不明ですが おそらく何キロにも及ぶので 潜水艦などを氷の下に潜らせて探査するのは 非常に高価で 極めて難しいのです