Overview
I will tell you the EXACT steps to getting to Sans’ “secret’ lab!
Getting Started
So the first thing you need to do is get to the judgement hall. I’m pretty sure this only works on a True Pacifist run. After getting to the Judgement Hall, SAVE, and then you do have to listen to Sans’ judgment. When the judgement is finished reload your save by quitting and running Undertale again. Then go talk to Sans again. You need to do this process a total of 3 times in order to get the Silver Key. Before i countinue I need to apologize for the lack of photos.
Getting The Next Key
The next key is found in Sans’ room which is the room in Sans’ and Papyrus’ House. By obtaining the Silver Key from telling Sans that you’re the legendary fart master, you now have acess to Sans’ room. In his room you find nothing but darkness and after sometime of seemingly walking forever but going nowhere Papyrus turns on the lights and relizes that Sans was just pranking you “From across time and space.” Now you can see a self sustaining tornado and the normal furniture yo would see in a bedroom,as well as a pile of socks and a lamp with a flashlight,with no batteries, stuck into the lightbulb socket. If you interact with the dresser you get the next key.
Lastly, Getting to the Lab
Now you have the key to Sans’ Lab. To enter simply go behind the house and press [ Z ].
There you can find all the stuff that makes people think,thanks to MatPat from GameTherory, that Sans is Ness from Earthbound. I mean its true that there is some good support behind that thereory but still I would like to know everyone else’s opinnions.