Anyone who moves to Boston can expect to be told within the first few weeks that they really should check out the Mapparium.

It's customary to nod and express thanks for this suggestion, but it's often years before those newcomers bother to ask: "What on Earth is a Mapparium?"

The Mapparium is a three-dimensional map of the Earth, free from any kind of cartographic distortion because it is a sphere, albeit one visitors can walk through. Located in the Mary Baker Eddy Library, part of the vast Christian Science Plaza in Boston's Back Bay, it's three stories high and was finished in 1935, so its country names and borders are based on Rand McNally maps of that year.

The Mapparium comprises 608 stained-glass panels, each one representing 10-degree divisions of latitude and longitude. It’s now illuminated from behind by a sophisticated system of LED lights but was originally lit by 300 40- and 60-watt electric bulbs, which generated uncomfortable levels of heat and needed regular replacement.

To get inside the globe, you join a tour, which start every 20 minutes. They run every 20 minutes during operating hours, daily between 10 a.m. and 5 p.m. You walk through the Mapparium on a glass bridge that's 30 feet long.

The Mapparium — originally called the Glass Room or the Globe Room — was the creation of Chester Lindsay Churchill, the architect who designed the Christian Science Publishing Society building, home to the Christian Science Monitor newspaper. Mary Baker Eddy was the originator of the Christian Science spiritual movement and founded the Monitor in 1908.

Because the Mapparium is made of glass — and, thus, doesn’t absorb sound — you are in a whispering gallery. The volume of your voice is strikingly amplified at the center; if you are near the edge and whisper softly, someone on the opposite side will hear you with ease.

The Mapparium: Hours: Daily, 10 a.m. to 5 p.m. Admission: Adults, $6; seniors (62), students, and youths (6-17), $4; children (5 and under), free. Information: (click Exhibits) or 617-450-7000.