Although I don't have the "official" and definitive answer, (and someone who does will probably address this momentarily), here's my guess.
A large number of posters here have either stopped posting and had their accounts become inactive, or fucked up and got banned.
These long-gone posters made posts that became important and integral parts of threads that are too "good" simply to delete, but for various reasons it isn't needed or appropriate to include the actual (former) poster's user name.
So, rather than delete the posts, or threads, their (inactive or banned) user names have been changed to *donotdelete*, allowing the content of their posts to remain embedded in threads that should remain as part of the permanent LTTD archives, while the user names themselves are acknowledged as irrelevant.
Think about it.
Helium II is a superfluid, a quantum mechanical state of matter with strange properties .
The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and hundred of times that of copper. This is because heat conduction occurs via a quantum mechanism.
Second sound is a quantum mechanical phenomenon in which heat transfer occurs by wave-like motion, rather than by the usual mechanism of diffusion. Heat takes the place of pressure in normal sound waves. This leads to very high thermal conductivity. It's known as "second sound" because the wave motion of heat is similar to the propagation of sound in air.
Sound waves are fluctuations in the density of molecules in a substance; second sound waves are fluctuations in the density of phonons. Second sound can be observed in any system in which most phonon-phonon collisions conserve momentum. This occurs in superfluids and in dielectric crystals when Umklapp scattering is small.