As of update 46.3, the Secret Elemenets menu was removed. It was previously displayed with the keybind RAlt + RCtrl. The elements from it still exist, however the only means to access them is with the search menu, PROP, the console, copy/paste, or selecting them with the middle mouse button.

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 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.

32px 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 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.

DYST

This element is dead yeast. Yeast dies and turns into this when it is heated too much, or if Neutrons (NEUT) touch it. If DYST is heated up even more it will turn into Dust.

SHD 2-4

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

EQVE

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