Went early in the morning in late December 2012. We only had 20 people on our tour. The tour guide was very knowledgeable about the Biosphere 2 - both the first and second mission - and the current mission - LEO which is being put together to start shortly. The building is amazing and love that Arizona State University is continuing the use of it to understand more about the planet (Biosphere 1). There are many stairs, but it is not taxing. The total walking trip is about 1 1/2 hours - and you do walk that long. Very informative.
Go early in the morning - it opens at 9 - get there as close to that as possible. The tour people told us that the tours in the afternoon have many more people in them then the morning.
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