That is, the Sackler Gallery, the Ripley Center, and the African Art Museum. Does anyone know the story behind these? It seems like an extreme solution to what I imagine was a space issue.
Found this, FWIW. It doesn’t really explain how they came to their decision, but I would assume that the possibility of fitting three stories’ worth of museums underneath a single 4 acre former parking lot had a lot to do with it.
They couldn’t very well build a modern three-story above-ground building right next to the Castle. It would have blocked the view, and would never have been permitted. Building underground was the best option for using the available space.
I used to have an office in the Ripley Center. We called it “The Basement of the Nation’s Attic.” 
I didn’t really care very much for working down there - no windows, so you could never tell what the weather was like. Later on I got an office in one of the towers of the Arts and Industries Building, reached via a spiral cast-iron staircase. That was the coolest office ever - if a bit drafty.
From a preservation standpoint, and underground enviornment is easier to control. The humidity and temperature remain fairly constant, which is important to preserve artifacts.
In the museum where I work, we have a refrigerated vault for our more fragile items. It pretty much mimics the conditions one would find in a basement: cool and about fifty percent humidity. of course, it being on the second floor, we have to have a lot of expensie equipment to keep it that way.
Because they didn’t want to alter the majestic vista that is The Mall?
Actually Spiff is partially correct.
The Mall is certainly an important part of DC’s cultural and historic landscape, and whatever can be done to avoid cluttering it up with more big buildings shoudl be avoided (especially by institutions like the Smithsonian, with a mandate to protect heritage).
A great example of how a low-impact visitor centre can be pretty effectively added to a cultural landscape is the Head-Smashed-In buffalo jump in Alberta, Canada. For nearly 6,000 years, First Nations people used the cliffs to harvest buffalo. It’s now a UNESCO World Heritage site (as well as a National Historic Site of Canada), with the interpretive programs run by the local Blackfoot band.
http://www.seevirtual360.com/themes/2/theme02.aspx?listingID=5027
(Choose “front view,” and you’ll see how smoothly they built the visitor centre into the hill itself).
The complex is behind the Castle, on the Independence Avenue side of the Mall. Any building there would not really have been visible from the Mall, being blocked by the Freer Gallery and the Arts and Industries Building as well as the Castle, and would not have impacted the vista much. It would, however, have blocked the view of the Castle from Independence Avenue.
At the time it was built, this was one of the last available spaces on the Mall. The only other space was the one where the American Indian Museum has since been built. I don’t know why that one wasn’t used, but it may have been because they wanted to have additional office space right next to the Castle.
The apparently open space west of the Freer is actually occupied by the Department of Agriculture, and the space between Natural History and the National Gallery of Art is not available because it would block the view of the National Archives on Consitution Ave.
sigh Colibri worked at the Smithsonian? Kewl! I was up for a job at Chicago’s Museum of Science and Industry but didn’t get it Instead I’m completely out of the museum biz, making more money closer to home and didn’t have to relocate a rusty, old submarine that they were worried would buckle if they moved it. Less stress but no glory.
I still work at the Smithsonian, but at their Panama tropical research center now.
I spent a couple of years as a free-lance exhibition consultant in Washingtion, for exhibitions at SI and elsewhere. (I like to tell people I spent two years as an exhibitionist on the National Mall, and got paid for it. ) Then I worked in Smithsonian offices on the Mall for a couple of years before transferring down here. When I’m not doing ornithology, I still work on exhibitions. I’m currently exhibition curator for Panama’s planned Museum of Biodiversity, designed by Frank Gehry.