MSHA - Code of Federal Regulations - 30 CFR 57.4104
30 CFR § 57.4104
(a) Waste materials, including liquids, shall not accumulate in quantities
that could create a fire hazard.
(b) Waste or rags containing flammable or combustible liquids that could
create a fire hazard shall be placed in the following containers until
disposed of properly:
(1) Underground--covered metal containers.
(2) On the surface--covered metal containers or equivalent containers with
flame containment characteristics.