Yeah I remember them as pods where mako waste was compacted into materia. Didn't know where I got that from
You could also have gotten it straight from the game.
Script from PS1 version:
Sephiroth
“…now I see, Hojo.”
“But, even doing this,
will never put you on the same level
as Professor Gast.”
Sephiroth
“This is a system that condenses and
freezes the Mako energy…
that is, when it's working correctly.”
“Now…what does Mako energy become
when it's further condensed?”
Cloud
“Uh, umm……Oh yeah!
It becomes a Materia.”
Sephiroth
“Right, normally.
But Hojo put something else in there.
…Take a look.”
Cloud
“Wh…what is this!?”
Sephiroth
“Normal members of SOLDIER are humans
that have been showered with Mako.”
“You're different from the others,
but still human.”
“But, what are they?
They've been exposed to a high
degree of Mako, far more than you.”