The ‘Underground Lake’ and Underground Passageway are a secret entrance to the museum.
This section of the museum grounds is a path utilized by Professor Windlenot as a private entrance to the museum. Since the building is also his home, a discrete entrance like this is understandable. It is in three different sections, and each one will be discussed.
The first part is the tunnel that connects the stairway under the Stonehenge Display. The tunnel is straight forward, and lit up by lights powered by a switch by the door. The only time you have to choose a path is when the underground tunnel has to go around something. The two paths meet back up, and then go down a few stairs before finishing at a door that leads to the second path. There is a light switch for all the lights in that tunnel right near the door from the Stonehenge Display.
The second part is the underwater lake. One enters this room on the one shore of the lake. The first thing one sees when they enter the door is an Egyptian River Boat. Looking around at the shore line leads one to discover the corpse of Professor Windlenot, still clutching the book, Egyptian Heiroglyphics Explained. When that is done, one just needs to get in the boat, release the anchor mechanisim, and move forward.
Once the lake is crossed, there is a small pot next to an steel brace structure. This is the vessel the Stone Ixupi should be in. Once it is opened, Professor Windlenot’s spirit comes out of it.
About three quarters across the lake, the first Ixupi attack happens. It come from the Water Ixupi. This attack cannot be avoided, no matter what. It is best to get to the other side right after the attack happens.
The third part is the passageway, a long corridor with few turns. It is the path that leads from the lake, to the elevator to Professor Windlenot‘s Office. It is right by the elevator where the only true puzzle in this area exists.
There are three things amiss in this area. The first one is the fact that Professor Windlenot‘s corpse is on the shore closest to the way out of the museum. . The second is the fact that the Stone Ixupi’s vessel is on the ground on the other side of the lake. The last thing that is amiss is the camera just by the elevator door. The thing makes sure it is watching you when you approach the elevator. This could be the sign that someone, or something, is in control of the camera.
There is one thing that bothers me about this area, and it is something that takes speculation. When one enters the lake part of this area, the boat is on the same side as you. This is obviously because Professor Windlenot had brought it over. When Beth and Merrick entered, they used the boat to get over to the other side. That meant the Professor couldn’t have used it to get across the lake when he returned to the museum.
One could speculate that they didn’t take this route in, but all signs point to this conclusion. This would mean that there is some special mechanism in play. When one enters through the Stonehenge Display, some mechanism brings the boat to the proper shore. It is either this, or Professor Windlenot, at the time of his return, took the main entrance into the museum.