Skil

An illustration of a skil on a volcano. (TV: The Power of the Doctor)

You may be looking for skill.

According to a diagram used by the Spy Master, a skil was a part of a volcano. (TV: The Power of the Doctor)

Behind the scenes

Although "skil" is not the name of a real part of a volcano, it appears to be a misspelling of "sill", a volcanic feature located in the same location as "skil".