My personal observation, after digging underneath Elder altar many, many, many times on many servers, is that starting along one edge of the altar, digging straight down (the 8m it allows), that I don't always end up with a completely level, flat floor... I have had differences of several meters, meaning I have had spots that were not the full 8m deep, as if there were an uneven "bedrock" layer under the terrain.

notice on the left side, in the far corner, the floor starts sloping up...

https://steamcommunity.com/sharedfiles/filedetails/?id=2855621495

Obviously, under the altar, the dirt was not sticking up through the altar floor, so why there is an incline underneath, is unknown. Sometimes I get a completely level flat floor at the same altar on at least 13 maps of the same seed. I did this many times on the same seed to experiment with various Elder altar base builds. Sometimes there's sloped ground and sometimes it's flat.