Classically, it is forbidden to escape, but according to the (then) newly discovered principles of quantum mechanics, it has a tiny (but non-zero) probability of "tunneling" through the barrier and appearing on the other side to escape the nucleus.
Historically, escaping was prohibited, but according to the recently discovered principles of quantum mechanics, there is a small but definite chance that particles can "tunnel" through barriers and emerge on the other side, thus escaping the nucleus.