some general facts, in case someone is not aware:
- the upper left room on the map (the one with the disorder portal) has the coordinates (0,0) and the lower right one is (47,29).
- many of the locations are quite hard to get to and are related to the achievements, so you need to find any KU achievement guide on your internets (www.playstationtrophies.org/forum/knytt-underground/184477-knytt-underground-trophy-guide-roadmap.html for example, i have no idea if posting such a link here is okay, though i haven't found any ban in the rules)
- there is no "Heroic Dance" achievement in the pc version (and i have killed N time to understand this), but there is "Pew Pew! - LAZA KNITEZ" achievement instead.
- there is a full map of under-undergrounds on this cool picture below.
(http://i1.minus.com/i9gcHeT4LHwt5.png) (http://minus.com/i/9gcHeT4LHwt5)
...or the same picture is attached to the post.
okay, sooo... the sorta-guide for the secret locations is below. obvious non-secret locations like Disorder, Demo and Interlude are not described. the 3rd chapter is considered unless noted otherwise.
1) Entrance to Red Forest, Red Forest - the secret areas in The Figure secret area. the entrance to The Figure is at (38,29) under the column. see Falu Kohe and Mysterious Forest achievements in the achievement guide.
2) Hi Ponto secret in Interlude. the entrance is in the first room of Interlude (19,34) in the upper right corner. it is quite hard to get there, as you need a powerup located two rooms away. a secret path to Hi Ponto room itself leads from the empty room in the lower left wall.
3) a secret in Interlude, the entrance is up above in (20,31). see Fireworks achievements in the achievement guide.
4) there is a nested secret in this secret, a path to the left in (24,32) in the middle of the left wall.
5) a secret in the last room of Interlude (19,31), the entrance is up above. see Dance In The Sky achievement in the achievement guide.
6) Juni and The Puzzle Piece. this is a secret in Credits. the path begins in the first room of Credits (21,34), you need to go via upper path 3 rooms to the secret path in the ceiling and to the room with the teleport (23,33). see Knytt Stories achievement in the achievement guide.
7) there is a nested secret in this secret, there is a path in (28,32) in the ceiling of the cave. see Tobias Chilli achievement in the achievement guide.
8) there is a secret in (33,29) in the ceiling above the left column, see Sandwich achievement in the achievement guide.
9) a secret with an earthquake button. a secret path starts in (19,29) in the center of the left column. the earthquake opens the path to Mini Figure in (39,29). see La Vupene achievement in the achievement guide.
10) there is a nested secret in this secret, in the earthquake button room (19,37) there is a secret path down on the right wall.
11) a path is opened in (38,28) after the earthquake to Mini Figure in (39,29). see La Vupene achievement in the achievement guide.
12) a secret final. there should be Enigma in the inventory when the last bell is rung. see What She Longed For achievement in the achievement guide.
13) start menu area, there is a secret path in the Erase saved data room behind the Chapter 1 room. the path is on the ceiling above the teleport, exactly where the beam points. the path leads to (42,35) with the teleport which leads to The Toast/Rainy Tower area (27,42).
14) start menu area, on the left of the room with the game name there is a room where the game starts for the very first time. in the room below(38,35) there is a secret path in the upper right corner which leads to the debug teleport. there is another secret path from this room to the sheep, see Because it’s tradition achievement in the achievement guide. there is another secret path from the sheep to the room with a spiral and there is nothing in this room (of which i am not sure. any ideas?).
15) the debug teleport leads to Chowdren area where some guy (never got any idea who he is) tells random boring stuff about the game.
16) a secret path in the upper left room (10,39) in Chowdren in the upper left corner leads to 3 icons of Juni, that-tail-with-egg-girl and who-tf-is-she.
17) there is a secret path in (30,26) in the upper left, see Redundant Bird achievement in the achievement guide. this exits in (15,2).
18) there is a secret path to Susanna's house in (45,18) in the upper left, see Habanero Apologize achievement in the achievement guide. this exitst in (2,7).
19) start menu area, a secret path leads down from the room with 3rd chapter teleport. a teleport to the Neon Fall area (see below) is at the end of the path.
20) immediately left from the entry to this secret there is a path to a nested secret, a teleport to HUMLab Murder Mystery (38,37), another impassable challenge.
21) there is a secret path from this secret to a nested secret, a teleport to Sync Transform (21,36), another impassable challenge.
22) a teleport at the end of the secret path from the 3rd chapter room leads to Neon Fall (14,39), one of the biggest secret area.
23) a nested secret in Neon Fall in (22,40), a secret path in the ceiling in the middle of the descent. if passed to the end, not taking the other secret paths, there is "Pew Pew! - LAZA KNITEZ" achievement in the room (21,38) before the last room. the path is quite hard to pass, it is another impassable challenge. a teleport brings back to Neon Fall in (22,41).
24) there is a nested secret in the nested secret in Neon Fall in (24,39). the secret path is in the ceiling on the left, the exit is back to Neon Fall.
25) a nested secret Tiny Figure in a nested secret in Neon Fall in (24,37). the secret path is in the ceiling in the middle. another impassable challenge. there is a disco in the end and a teleport to the beginning of Tiny Figure.
26) a secret three-ways crossing in (8,38). there are 3 entrances to it - a secret path in the left wall in (35,6) is very easy to take, a secret path on the ceiling on the right in (8,24) and a secret path in the ceiling in the middle in (44,22).
27) a secret path to the museum of arts starts in this three-ways crossing in the ceiling in the very left corner, near the upper left entrance.
28) a secret path to Ethan's museum is in (23,20) through the crack on the ceiling to the left. this crack is high above, i have no idea how to get there without cheating, some mad bouncing skillz, probably. the museum itself is located in (43,40).
hmmm... something like this.
there are couple of rooms i have no idea about. i'm very interested if someone can shed any light on these (well, there is at least one such a guy, i guess).
?1) black hole (Sun, Won't you come) is located in (22,36), the exit leads to (5,9) so it is like instead of save-point room in (4,9) but i have no idea how to get there at will.
?2) the area in (16-18,37) is marked on the map as Unknown Disorder. it looks like Disorder and i can imagine it was used in some dream or vision during the storyline, but i cannot recall anything alike.
i also attach here a text of the achievement guide mentioned above, as a backup, no links, no video though.