An underreamer for increasing a diameter of a wellbore, comprising: a body having an axial bore extending at least partially therethrough; an electromagnetic activation system (210, 500) disposed at least partially within the axial bore of the body; a valve (340, 520) disposed within the axial bore of the body and coupled to the electromagnetic activation system (210, 500), the valve (340, 520) including a mobile element (414, 522) and a static element (450, 526), the mobile element (414, 522) being coupled to the electromagnetic activation system (210, 500) and being arranged and configured to move from a first position where the mobile element (414, 522) obstructs fluid flow through the valve (340, 520) to a second position where the mobile element (414, 522) permits fluid flow through the valve (340, 520); a flow tube coupled to the valve (340, 520), the mobile element (414, 522) of the valve (340, 520) obstructing fluid flow through the flow tube in the first position and permitting fluid flow through the flow tube in the second position; a mandrel coupled to the flow tube, the mandrel having a first opening formed radially therethrough in fluid communication with the flow tube, the mandrel also having a second opening formed radially therethrough; a sleeve disposed radially-outward from the mandrel and adapted to move axially with respect to the mandrel from a first position to a second position when the fluid flows through the valve (340, 520), the flow tube, and the first opening in the mandrel, the sleeve blocking fluid flow through the second opening in the mandrel when in the first position, and the sleeve allowing fluid flow through the second opening in the mandrel when in the second position; and a cutter block movably coupled to the body and configured to move radially-outward as the mobile element (414, 522) moves from the first position to the second position. a body having an axial bore extending at least partially therethrough; an electromagnetic activation system (210, 500) disposed at least partially within the axial bore of the body; a valve (340, 520) disposed within the axial bore of the body and coupled to the electromagnetic activation system (210, 500), the valve (340, 520) including a mobile element (414, 522) and a static element (450, 526), the mobile element (414, 522) being coupled to the electromagnetic activation system (210, 500) and being arranged and configured to move from a first position where the mobile element (414, 522) obstructs fluid flow through the valve (340, 520) to a second position where the mobile element (414, 522) permits fluid flow through the valve (340, 520); a flow tube coupled to the valve (340, 520), the mobile element (414, 522) of the valve (340, 520) obstructing fluid flow through the flow tube in the first position and permitting fluid flow through the flow tube in the second position; a mandrel coupled to the flow tube, the mandrel having a first opening formed radially therethrough in fluid communication with the flow tube, the mandrel also having a second opening formed radially therethrough; a sleeve disposed radially-outward from the mandrel and adapted to move axially with respect to the mandrel from a first position to a second position when the fluid flows through the valve (340, 520), the flow tube, and the first opening in the mandrel, the sleeve blocking fluid flow through the second opening in the mandrel when in the first position, and the sleeve allowing fluid flow through the second opening in the mandrel when in the second position; and a cutter block movably coupled to the body and configured to move radially-outward as the mobile element (414, 522) moves from the first position to the second position.