A system for containing an undersea oil well spill, comprising: a containment housing having an open bottom configured to fit over an oil well spill pipe which extends into the housing,through which effluent comprising oil and/or gas is spilled into the sea, wherein the housing is lockable or sealable to a platform or ring surrounding the oil well pipe or is positioned on the sea floor; a pipe positioned and extending internally of the containment housing, into which the oil well pipe can be fitted, and effluent from the oil well pipe moves in operation, wherein the internal pipe is in fluid communication with an evacuation pipe which extends from the containment housing; characterised in that the system comprises: an evacuation pump for evacuating the oil spilled from the well at a rate approximately at least as great as the rate of the oil spillage from the oil well spill pipe creating an approximately equal or negative pressure environment within the housing, such that there is no buildup of pressure of the escaping effluent and gas within the housing; and an infusion line, and associated pump, connected to the containment housing for pumping selected sealing material into the housing, thereby sealing around the spill pipe within the housing, while allowing oil or other effluent to continue to be pumped out of the housing through the internal pipe and the evacuation pipe. a containment housing having an open bottom configured to fit over an oil well spill pipe which extends into the housing,through which effluent comprising oil and/or gas is spilled into the sea, wherein the housing is lockable or sealable to a platform or ring surrounding the oil well pipe or is positioned on the sea floor; a pipe positioned and extending internally of the containment housing, into which the oil well pipe can be fitted, and effluent from the oil well pipe moves in operation, wherein the internal pipe is in fluid communication with an evacuation pipe which extends from the containment housing; characterised in that the system comprises: an evacuation pump for evacuating the oil spilled from the well at a rate approximately at least as great as the rate of the oil spillage from the oil well spill pipe creating an approximately equal or negative pressure environment within the housing, such that there is no buildup of pressure of the escaping effluent and gas within the housing; and an infusion line, and associated pump, connected to the containment housing for pumping selected sealing material into the housing, thereby sealing around the spill pipe within the housing, while allowing oil or other effluent to continue to be pumped out of the housing through the internal pipe and the evacuation pipe. an evacuation pump for evacuating the oil spilled from the well at a rate approximately at least as great as the rate of the oil spillage from the oil well spill pipe creating an approximately equal or negative pressure environment within the housing, such that there is no buildup of pressure of the escaping effluent and gas within the housing; and an infusion line, and associated pump, connected to the containment housing for pumping selected sealing material into the housing, thereby sealing around the spill pipe within the housing, while allowing oil or other effluent to continue to be pumped out of the housing through the internal pipe and the evacuation pipe.