The sarcophagus lid
The object in question was found in 1952 inside the tomb chamber of the Temple of the Inscriptions as the sarcophagus lid of K'inich Janaab' Pakal, a ruler of the Maya polity of Palenque.
This drawing, made by Merle Greene Robertson in 1975 (source) depicts the carvings on the lid (you can click on it for larger version):
Note: since Mrs. Robertson used her first husband's name to sign her artwork, this picture is referenced as the Greene drawing in the document.
There was and is a lot of discussion within and outside the scientific world about the exact meanings of the symbols found on the lid. Opinions range from showing Pakal lying on top of the "earth monster" to portraying an ancient astronaut sitting in a rocket device.
Own theory
I myself too tried to decipher the pattern of the lid, and I hope the results presented in this document could be of some interest to the Internet (and perhaps even to the scientific?) community.
It was not mainly the human figure and the large shapes that grasped my attention (and in fact I even did not try to interpret most of them), but the small three-circled elements ("triplets") scattered seemingly without any purpose all around the upper part of the lid. I eventually came to the conclusion that these items are not (pure) decorations, but they were placed at their positions with clear intentions, forming together (along with the enclosing context) a very sophisticated method to encode a message about an interstellar travel from another solar system to the third planet of our one (i.e. the Earth).
This theory certainly sounds a bit (?) fantastical, but having spent a lot of time on the subject I'm quite confident by now that it could be fundamentally correct (even if not in every single detail), and may deserve to get publicized in some way. Thus, I have put these pages together, and would like to invite anyone interested in this subject to take some time and go through the decoding steps to form his/her own opinion about the matter.
The decoding process in 12 steps
You can start the route at the first step and then go through the steps one after the other by clicking the "Continue to the next step" link at the bottom of each page (recommended for the first reading). Or you can choose any of the links from the list below to go directly to a particular step.
You can also use the arrows at the top right corner to move across the document. They are also accessible by the corresponding access keys: P for previous, U for up and N for next (if you need help with using access keys in your browser please refer to this guide).