What kind of wire can I run underground without conduit?

UF-B cable is the wire made for underground runs without conduit: a solid thermoplastic jacket rated for direct burial, used for branch circuits to sheds, posts, and pumps. Service and feeder work uses USE or URD conductors the same way.

Direct-buried cable goes 24 inches deep in most cases, versus 18 for PVC conduit; the shallower 12 inch allowance covers GFCI-protected residential 120V circuits on breakers of 20 amps or less.

Romex cannot go underground under any arrangement, including inside buried conduit, and plain THHN needs a raceway everywhere.

Where cable rises from the trench, it still needs physical protection, so short conduit sleeves finish most direct-burial jobs anyway.

Depths and sizing for buried feeders live in the underground feeder calculator.