Despite its appearance, this is a singular mass noun. It does have a plural, for talking about a group of groups, which is spelled identically. You may also see a false singular form, congery, but this is frowned upon. But singular verb -- and on this shelf here is a congeries of origami. Adopted in the 1610s from Latin congeriēs, heap/mass/pile, from congerere, to carry together (from con- + gerere, to bear/carry) + -iēs, a nominalizing suffix (which also appears in rabies and series).
"A congeries of bubbles floated through the air."