From ArborWiki

Mushroom Park is a 1.54-acre neighborhood park located between Waltham and Saxon off of Scio Church Rd. The playground is nestled among berms, and among the berms are a few small concrete mushrooms. There is also a picnic table and benches.

http://www.a2gov.org/government/communityservices/ParksandRecreation/parks/Features/Pages/Mushroom.aspx