We have just learnt that "穴" means a "Cave" or "Hole" or "Burrow" and "九" has its original meaning as "crawling around to Explore or Search". So "究" was created with meaning of "Crawling in a cave to Explore or Search the unknown space". "穴" here is used to emphasize an unknown space that is being Explored or Searched.
so "究" means Explore or Search.
Below is the evolution history of "究". (image get fromhttp://www.vividict.com):
|Learn one Chinese Character a day - 究|
At the end of our lesson today, let us enjoy a Chinese Calligraphy with "究" inside: