Conversation so far:
When did the renovations take place?
The Merchandise Mart was modernized in the late 1950s and 1960s.
What did the renovations entail?
The Indian chiefs were removed and replaced with concrete plates in 1961.
How much did it cost?
After a 10-year, $100 million modernization in the late 1980s.
Have there been any other recent renovations?
The project was completed in 1991.

Question: What kind of expansions have they done?
Self-contained rewrite: What kind of expansions have been done to Merchandise Mart?