The old man tried to hide the money underground.

Literal

Old-man [topic-は] money [object-を] underground [in-に] hide [attempt-おうとした].

~おうとした (volitional + とした) means 'tried to, attempted to.' 地中に ('underground') specifies the hiding place. The ~(よ)う + とする pattern is the standard way to express an attempt in Japanese.