These elements are not in any menu. Most of them are alternate forms of other elements. There was a secret elements menu, accessed by pressing Alt+Ctrl, but it was removed in version 46.3. Currently, they can be accessed from the search menu, the console, PROP, copy/paste, or by selecting them with the middle mouse button or by pressing 'e'.

MORT

"Steam train." After being placed, it moves either left or right, leaving behind a very thick trail of SMKE.

LOVE

A solid element that forms instantly into the shape of a heart and is colored pink.

When DEST or WARP is dropped on love, more love is created :D Also LOVE has a starting temperature of 99.85 degrees Celsius, while most substances start at 22 C.

LOLZ

Similar to LOVE, but takes the form of the letters "lol" and is colored dark green. It also has the same starting temperature of LOVE. Just like LOVE, destroying it with DEST or WARP only makes more of it.

FRZW

This element is a liquid form of FRZZ, essentially WATR below 0 degrees but not ice. It can be created by careful manual cooling or mixing FRZZ with WATR.

BIZG

BIZR in gas form, created by cooling BIZR.

BIZS

BIZR in solid form, created by heating BIZR.

BRAY

BRAY is created from SPRKed ARAY. The BRAY element rapidly fades after its creation with a life of 30 frames. If two BRAYs collide, they will form a "solid" BRAY where they intersect that has a life value of about 1000 frames. BRAY can be colored using FILT. There is also another type of BRAY that is created when the ARAY is sparked by PSCN, it is brown and has a life of only one frame.

SPWN

Functions as a spawn point for STKM if it dies. If more than on is placed on a map, STKM will only respawn on the first one that was placed.

SPWN2

Functions as a spawn point for STK2 if it dies.

Only element in the game that is 5 characters long

DYST

Dead Yeast. YEST turns into this when it is heated too much, or if NEUT touches it. If DYST is heated up even more it will turn into DUST.

SHD2, 3, and 4

Created when SHLD is Sparked. SHD3 and SHD4 will regenerate particles of SHLD and SHD2, respectively, when there is nothing surrounding them.

EQVE

Equal velocity dust. Originally a test for moving solids, this powder currently does not do anything, is fairly strong, and may be replaced in the future.

EMBR

Ember. EMBR are just sparks that form when BOMB explodes. In bigger groups EMBR can light up stuff like OIL for example.

PSTS

Paste solid. Solid form of PSTE. Formed when PSTE is exposed to pressure.