Miss December, Last Known PlayaMaya Centerfold of the Month
It's not a physical object, anymore than Advent or a football season or The Twentieth Century are physical objects (though many--including Mayan Calendar Girls--depict the familiar stone Aztec Sun Calendar). It is the master calendar of pre-Columbian Central America, and represents an ordering of time tha is, like the Catholic calendar, as much a spiritual map as a chart of days.
Part of the appeal of the calendar is its elegant design, which is often described as gears, with each day in the short and long calendar being gears, as depicted in this illustration. It's a handy way to think of it and a precise calculator of date. It divides the year by a 13:20 ratio.
THE TZOLKIN CALENDAR
The Tzolk'in calendar works by the interaction of the larger 20 "tooth" "wheel" and the smaller 13 "tooth" "gear" to produce each unique "kin", or day.
It should be obvious that a calendar of 260 days is not sidereally accurate or of much use for things like agriculture. It's seen as a purely priestly sequence.
THE LONG COUNT (BAKTUN)
The "Long Count", a huge "cogwheel" in the calendar system was introduced much later, probably to deal with longer stretches of time once dynasties were established. It is the entire permutation of tzolkin and long count dates that yield the "limits" of the Calendar: a period of about 5000 years stretching from 3100 to 2012.
This triad of glyphs pictorially represents the date that the Mayans would mathematically express as "18.104.22.168.0.". It's the glyphs for the last day of the calendar.
The limestone shelf of the Yucatan peninsula is riddled like Swiss cheese. When large cavities are located near enough to the surface to be accessed they almost always have water in the bottom, creating a natural well cavern. Sometimes the underwater topography of these cenotes is a series of interconnecting cavities and tunnels, visted by SCUBA tourists. The fame of cenotes generally comes from the cult of sacrifice practiced late in Mayan history and yeilding underwater vaults full of skeletons and precious treasure.
This is an underworld--the name means "place of fear", and is ruled by 12 Lords of disease, paranoia and death. They are evil conniving spirits against which the Mayan heroes struggle. There are caves in Belize said to be the entrance to Xibalba. The K'iche people also apply the term to the dark slit in the middle of the Milky Way, a telling identification since the sun is located on that cleft at the end of the calendar.