Bleach Chapter 106 Translated - Spoilers!

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At the 2nd division, it was reported to Yoruichi that Urahara had headed to "the Maggots Nest". Soi fon was going to oppose it but Yoruichi told her not to worry and that Urahara must know what he is doing if he is taking his vice captain to that place.

Urahara eventually manages to drag Hiyori along with him. While walking there he tells her the history of the secret mobile corps. It was established separately from the Gotei 13 and the Shihouin family has a lot to do with it. Therefore the leader of the Shihouin family is given the rank of captain in one of the Gotei 13 divisions, that division will have a very close relationship with the secret mobile corps.

The current leader of the family is Yoruichi who is the captain of the 2nd division. The members of the division also have to take up a position in one of the 5 sub-divisions of the secret mobile corps. Urahara was included in this, the sub-division he once took responsibility for is the 3rd sub-division of the secret mobile corps which is called "Kanritai" which means “the Penitentiary Department.”

One of this department's duties is to confine shinigami who have a high possibility of bringing danger to others, which makes them also considered an obstacle for the gotei 13.

Urahara further explains that it's to imprison "the cause of danger". He points towards the northern part of the 2nd division which has the large prison where these prisoners are kept. It is known as the underground for special imprisonment or, the Maggots nest.