From the Super Mario Wiki, the Mario encyclopedia
Once Luigi has cleared the Scale Chamber, Snake Chamber, and Jewel Chamber, the Central Chamber starts filling up with sand, allowing him to access a door near the top of the chamber. Once Luigi enters the secret passageway, he must head right, up the stairs. Halfway up the stairs, a pressure plate is triggered, causing the walls to start closing in. The walls follow Luigi up both sets of stairs, trapping him at the end of the passageway, where there are three vases and a flame. There is a torch in one of the vases Luigi must light with the flame. He must then use the torch to burn a large web covering a mural of him on the left. When Luigi investigates it, he lifts his arms to mimic the pose, activating it. The wall then flips and takes Luigi into the Altar Chamber.
At the start of the secret passageway, there are two vases and spiders which come down from the ceiling. There are a few coins and a gold spider in the vase in the northwest corner. While there is nothing in the other vase, behind it is a sideways symbol. Luigi can use the Poltergust G-00 to match it up with the other symbols, opening a cache in the western wall to reveal coins, bills, and Gold Bars. At the end of the passageway, there are three vases, two of which contain treasure and spiders. The other contains a few coins and a torch Luigi must light on the flame in the southeast corner.
If Luigi returns to the Central Chamber, it is no longer flooded with sand. Instead, there are ladders which take him up to the door leading to the secret passageway.
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