Approximately 15,000 of the 300,000 books in the university's library, have grown mold, the Chicago-Sun Times reported Friday.

The collection is said to be worth more than $1 billion.

"We are really looking to protect this collection that is irreplaceable and really a gem in the state of Illinois. People come from all over to look at this material," Associate University Librarian Tom Teper said.

The facility is set to be shut down Feb. 25 so the structure and its contents can be undergo an $8000,000 cleaning.

The mold was reportedly first noticed in November after high humidity outdoors caused moisture and temperature problems inside the building which has a 25-year-old ventilation system.