Definition of "sponginess"

Actually the job was almost the despair of those early pioneers, for the surface was a "quaking morass," the surrounding district was undrained, and no sooner than soil was deposited than it was swallowed up, with no apparent effect, for the sponginess of the land remained unaltered.

1934 February, G. W. Tripp, “How Nature Harasses the Railway”, in Railway Magazine, page 79