Many levels. One of the widespread new methods of saving space entails building several levels for burial [this exists even on Har HaMenuchos]. The burial surface is built over a concrete floor with a large quantity of earth. Each level is supported by columns, and the roof of each level serves as the burial area for the level above. The burial style resembles קבורת שדה (23), only it is made from many concrete floors. [Pictures taken with permission from the end of the wonderful sefer “משכן הנפש ”; credit to the mechaber.]
All poskim today who fear Hashem’s word rule that one should not use these kevarim for many reasons. It is a departure from the customary minhag of kevura, and the main reason is because although there is dirt between each kever, they hold it is not considered in-ground burial; rather, it is like burial on a roof or in a dirt-filled container (שו''ת מנחת יצחק ח''י סי' קכ''ב, הגריש''א בקובץ תשובות ח''ב סי' ס''ד, מכתב של בד''ץ עדה החרדית ). Even if they make a hole leading to a pipe filled with dirt that goes all the way to ground level, that does not make it like in-ground burial; it is like burial in an עציץ נקוב. Although the contents of an עציץ נקוב are considered connected to the ground regarding מצוות התלויות בארץ, that is because the plant inside gets nourishment from the ground. But for kevura, the idea is not to be connected to the ground, but to be buried in the ground. One posek even ruled that a meis who wanted traditional burial should be removed from a multi-level kever (מו''ר בעל קנה בשם, היכל הוראה ח''א סי' קצ''א ).
They also found instances of tremendous disgrace to the meisim. After it rained onto the upper level, all sorts of remains of the meisim came out of the drainpipe. These things are usually absorbed into the ground, but when a meis is buried on concrete, not in the ground, there is nowhere for the remains to be absorbed. This is the exact opposite of the purpose for burial.
