Civil Code (Cap. 16), article 442

442. ( 1 ) W h e r e a c i s t e r n e x t e n d s u n d e r t h e t e n e m e n t o f t h e neighbour, such neighbour may bore a hole and make use of the water, subject to his obligation to refund to the owner of the tenement in which the excavation of the cistern was commenced one-half of the expense incurred. (2) Each of the two neighbours may demand that the part of the cistern which exists under his t enement be separated from that existing under the other tenem ent by means of a wall to be constructed and, when necessary , repaired at joint expense. (3) Where, however, the part of the cistern existing under the tenement of one of the neighbours is considerably larger than t he part existing under the tenemen t of the other, the court may, according to circumstances, in ord ering the separation, direct the former to refund to the latter a proportionate part of the sum which he may have paid for the excavation of the cistern. Distance of windows, etc., from party-wall.