A method of determining the location of an acoustic source (100, 200), the method comprising: providing a distributed optical fibre sensor in an area to be monitored, the distributed optical fibre sensor being deployed in a known location; receiving one or more acoustic signals from the acoustic source (100, 200) at a plurality of sensing points (201, 202) spatially distributed along the length of the distributed optical fibre sensor such that each acoustic signal is received at the respective sensing points (201, 202) at different respective times to thereby obtain a first two dimensional spatiotemporal data set; and processing the received acoustic signals to determine the location of the acoustic source (100, 200) relative to at least one of the acoustic sensing points (201, 202) along the length of the distributed optical fibre sensor; wherein processing the received acoustic signals comprises evaluating a plurality of cost functions for one or more of the received acoustic signals, wherein evaluating each of the plurality of cost functions comprises: i) time shifting the first spatiotemporal data set in dependence on the positions of the sensing points (201, 202) along the length of the distributed optical fibre sensor at which the acoustic signal is received and an estimate value for the shortest distance between the acoustic source (100, 200) and the distributed optical fibre sensor to obtain a time-delayed two dimensional spatiotemporal data set; and ii) calculating the cost function based on the time-delayed spatiotemporal data set; wherein the processing further comprises selecting the maximum cost function from the plurality of cost functions, and determining the shortest distance between the acoustic source (100, 200) and the distributed optical fibre sensor based on the estimate value for the shortest distance used to calculate the maximum cost function. providing a distributed optical fibre sensor in an area to be monitored, the distributed optical fibre sensor being deployed in a known location; receiving one or more acoustic signals from the acoustic source (100, 200) at a plurality of sensing points (201, 202) spatially distributed along the length of the distributed optical fibre sensor such that each acoustic signal is received at the respective sensing points (201, 202) at different respective times to thereby obtain a first two dimensional spatiotemporal data set; and processing the received acoustic signals to determine the location of the acoustic source (100, 200) relative to at least one of the acoustic sensing points (201, 202) along the length of the distributed optical fibre sensor; wherein processing the received acoustic signals comprises evaluating a plurality of cost functions for one or more of the received acoustic signals, wherein evaluating each of the plurality of cost functions comprises: i) time shifting the first spatiotemporal data set in dependence on the positions of the sensing points (201, 202) along the length of the distributed optical fibre sensor at which the acoustic signal is received and an estimate value for the shortest distance between the acoustic source (100, 200) and the distributed optical fibre sensor to obtain a time-delayed two dimensional spatiotemporal data set; and ii) calculating the cost function based on the time-delayed spatiotemporal data set; i) time shifting the first spatiotemporal data set in dependence on the positions of the sensing points (201, 202) along the length of the distributed optical fibre sensor at which the acoustic signal is received and an estimate value for the shortest distance between the acoustic source (100, 200) and the distributed optical fibre sensor to obtain a time-delayed two dimensional spatiotemporal data set; and ii) calculating the cost function based on the time-delayed spatiotemporal data set; wherein the processing further comprises selecting the maximum cost function from the plurality of cost functions, and determining the shortest distance between the acoustic source (100, 200) and the distributed optical fibre sensor based on the estimate value for the shortest distance used to calculate the maximum cost function.