The Digital Sea is a liquid sea that resides below all of the four main sectors and is how the Lyoko program portrays the supercomputer's DOS or DOS's quantum computer equivalent. Another theory is that it portrays the trash folder. Aelita uses this to her advantage by dropping monsters into the Void using her Creativity. When something that shouldn't be in the Void falls into it, a beautiful white column of light emanates from it. It is the most forbidden place on Lyoko due to the deletion it causes.
There are many other theories about the Sea - one is that it is a visual representation of all the files housed in the computer - presumably if a person falls into it their data is decompiled and scattered among these files. Whatever it is composed of, it takes the form of an actual sea. XANA used this to his advantage in Tidal Wave when he managed to flood Lyoko in order to protect his activated tower and to keep anyone else from entering Lyoko until the problem was fixed. But of course XANA failed again.
The digital sea has various color differences between the different seasons. The digital sea in the forest used to be blue in season 1 and yellow in seasons 2-4, the desert sea used to be white in season 1 and a dark yellow color in seasons 2-4 and the Mountains' sea wasn't shown in season 1 because of the fog there, but was probably grey.
The Digital Sea is also a huge part of the Network. It flows between the "buildings" and hubs in the network, and some of XANA's monsters have been created specifically to survive in the digital sea.
The only members of Team Lyoko who have fallen into the Digital Sea are Yumi (Cruel Dilemma) and Aelita (Distant Memory). Yumi was rescued from the Digital Sea by Jeremie's one off materialization program (which Odd created by dropping candy on the keyboard); it is unknown whether Code: EARTH would have the same effect however it seems that it doesn't as when Aelita fell into the digital sea in Distant Memory, the code wasn't used to revive her. Due to the risk of eternal virtualization, the team continues to avoid the Digital Sea unless in the Skidbladnir. Aelita was saved by her father who composed himself as a purple blue orb that floated out of the Digital Sea and placed Aelita in the Ice Barrier before departing again.