referring the the drawing , fig1 and 2 disclose one embodiment of the invention wherein a tuned circuit , including an air - core inductance 10 with a capacitor 12 connected in parallel with the inductance , is carried by a buoyant carrier 14 with such combination enclosed in a spherical housing 16 , the housing 16 containing a quantity liquid 18 , such as water , sufficient to cause the carrier 14 to float freely within the housing 16 . it is the function of the buoyant carrier 14 to carry the tuned circuit in such a manner that the carrier and tuned circuit combination , when it is free floating within the housing 16 , causes the axis of the wire coil that provides the inductance 10 to assume a desired position with respect to the horizontal . the optimum position is with such axis vertical and in any event closer to the vertical than the horizontal . in the case of the embodiment shown in fig1 and 2 , the inductance 10 is provided by a coil of insulated wire having an air core , the coil presenting a generally flat profile . this being the case , the carrier 14 used in the structure of fig1 and 2 is disc shaped with the tuned circuit disposed within the carrier 14 . the diameter of the coil for the inductance 10 is slightly less than the inner diameter of the carrier 14 so there is an even distribution of the weight of the coil about the inner periphery of the carrier 14 causing the coil to be disposed horizontally when the carrier 14 and tuned circuit float on the liquid 10 . with the coil disposed horizontally , its axis will be vertically disposed to provide the optimum position for the coil of the inductance 10 . the carrier 14 of fig1 and 2 can be formed with a circular base member 20 that is cup - shaped . the base member 20 has a flat central portion at the periphery of which is an integral upstanding rim or edge 21 having an outwardly extending rim 22 . the tuned circuit is placed in this cup - shaped base member 20 with a flat circular cover member 25 positioned as cover for the member 20 . the cover member 25 is fused to the rim 22 using vibration or spin welding techniques . the cover member 25 can also be fused to the rim 22 by ultrasonic welding techniques . materials for the carrier 14 can be any of a number of well known plastic materials such as polyvinylchloride , polyethylene or acrylonitrile - butadiene - styrene , for example . the sphere 16 can be formed from two identical hemispheres 23 and 24 formed from a suitable plastic material such as high density polyethylene . the hemispheres are formed with a small outwardly extending flange to provide a surface at which the two hemispheres can be fused by the use of ultrasonic vibration or spin welding techniques . after the carrier 14 has been positioned within the sphere 16 and the hemispheres 23 and 24 forming the sphere have been fused together , a measured amount of liquid 18 is introduced into the sphere 16 via a hole ( not shown ) which is closed after the liquid is introduced . water can be used as a suitable liquid . the sphere 16 need only have a diameter slightly greater than the diameter of the carrier 14 when a liquid 18 level is used that positions the free floating carrier 14 to float at the diameter for the sphere 16 . fig3 and 4 disclose another embodiment of the invention that is like the embodiment shown in fig1 and 2 , except that the carrier 14 and tuned circuit combination are constructed differently . parts in fig3 and 4 corresponding to the same parts as have been described in connection with fig1 and 2 are identified using the same reference numerals as are used in fig1 and 2 . the structure in fig3 and 4 includes a sphere 16 , constructed using two hemispheres 23 and 24 in the same manner as described for sphere 16 of fig1 and 2 , a quantity of liquid 18 , a tuned circuit structure 30 and a carrier 32 for the tuned circuit structure 30 . the tuned circuit structure 30 includes an inductance provided by a ferrite core 34 about which an insulated wire is wound to form a winding or coil 36 . the ends of the wire for the winding or coil 36 are connected to a capacitor 38 . the structure 30 is completed by inserting the ferrite core 34 , winding 36 or coil and capacitor 38 assembly in one end of a tube 40 which is closed at the other end with the open end of the tube 40 then closed off with a cap which is fused to the tube 40 using spin welding techniques . the carrier 32 for the tuned circuit is formed from a circular , flat piece of buoyant material such as styrofoam in which a centrally located opening is formed for receiving the tuned circuit structure 30 to position the tuned circuit structure 30 so it extends from both sides of the carrier 32 . the tuned structure 30 is secured to the carrier 32 in a suitable manner such as by a wedge ( not shown ) inserted between the structure 30 and the carrier 32 or by use of a suitable adhesive . the heavier of the two portions of the tuned circuit structure 30 extending from the carrier 32 is received in the liquid 18 that is introduced into the sphere 16 after the carrier 32 and tuned circuit structure 30 has been positioned in the sphere 16 and the two hemispheres 23 and 24 making up the sphere 16 have been fused together . with the tuned circuit structure 30 centrally located in carrier 32 , the carrier 32 floats level with the surface of liquid 18 causing the axis of the winding or coil 36 to be vertical to provide the optimum position for the winding or coil 36 for detection of the marker after it is buried . the passive marker devices that have been described are usable as passive markers in carrying out the method for locating buried markers as described in u . s . pat . no . 4 , 119 , 908 to armond cosman et al . while the devices as described are directed to the attainment of an optimum position for the coil used in the tuned circuit , it should be understood that a device constructed so that the axis of the coil positioned by the freely floating carrier for the tuned circuit is closer to being vertical than horizontal is considered as being constructed according to the teachings of this invention . while only two embodiments of the invention have been described in connection with the drawing , it will be understood that various modifications may be made to such embodiments and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention .