Radar, sonar and more

I'm working on a scenario inspired by a recent report
I heard on NPR about the ocean of Europa ... it's
still in the embryonic stages of development.

Anyway, I need some help that I'm hoping some listers
might be able to provide:

No. 1 Is there any naturally occurring element that
can block radar? Is there any elemnet hypothesized or
in hard sci-fi literature that can do this?
Essentially what I'm looking for is something that can
refract radar and prevent accurate readings.

No. 2 Will an abnormally loud aquatic environment
prevent the effective use of sonar?

Thanks in advance for any help you can provide.
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