This appears to be an old municipal water-service access box/manhole, likely dating from the early 20th century. The two-part cover would have allowed utility workers to access components of the underground water system—such as a water valve, service connection, or meter assembly—without excavating the street or yard.
The unusual design is essentially a utility access hatch, not a household fixture. The smaller central lid could provide access to one component while the larger surrounding cover provided access to the larger chamber.
Because it’s an old installation, the exact function would depend on what is underneath it, but the “WATER” and Somerville municipal marking make a water-utility access point the most likely explanation.
