I just tried the following experiment on several different machines; 
several different recent versions of Sage; algorithm = ple, m4ri; and n = 
20000, 25000, 30000. In all cases, the result is identical:

sage: M = MatrixSpace(GF(2), n)
sage: for i in range(10):
...:     print M.random_element().rank()
19937
19937
19937
19937
19937
19937
19937
19937
19937
19937

Using the Internet, I also retroactively tested this in Sage 4.0.1. (Search 
in the page for "19937".)

https://wiki.sagemath.org/sagebeatsmagma

By contrast, I just tried a whole bunch of sparse examples coming from 
Cremona modular symbols that gave more plausible answers, e.g.:
sage: S = CremonaModularSymbols(300001, sign=-1)
sage: T = 
hecke_matrix(2).sage_matrix_over_ZZ().change_ring(GF(2)).dense_matrix()
sage: T.dimensions()
(27549, 27549)
sage: T.rank()
27461

I'm not sure if this rank is correct, but at least it's not 19937.

Kiran

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