We consider the problem of ubiquitous data collection by the mobile user in a wireless sensor network. The mobile user uses a handheld device to communicate with sensor nodes in the network. The mobile user roams within the network and instantly accesses network-wide data on a need basis. For instance, in the aforementioned GreenOrbs project, the mobile user needs to collect data from the entire network in many scenarios, such as the periodical measurement of concentration of carbon dioxide for the scientific analysis, the network abnormal event detection, etc the ubiquitous data collection problem studied in this paper is essentially different from conventional mobile sink-based data collection. The sink mobility has been introduced to benefit the data collection operation, e.g., to improve the network lifetime. The mobile sink is essentially cooperative to the data collection. We consider the ubiquitous data collection by mobile users in the wireless sensor network. Mobile users are equipped with handheld devices that communicate with sensor nodes and instantly access the network through nearby sensors. Such a pervasive usage of sensor networks explores n-situ interactions with human beings, provides people facilitated means of data collection, and thus significantly expands the capability of wireless sensor networks. On the other hand, there are a number of forest rangers patrolling around the mountain to detect any accidents in the forest, like the fire indication, the vegetation damage, etc. The ubiquitous data collection problem considered essentially differs from traditional data collection problems in static settings. In a static sensor network, an optimal data collection tree is usually built to collect the network-wide data. To efficiently deliver network-wide data to the mobile user, the data collection tree needs to be constructed or updated from time to time according to the mobile user’s movement. Directly adopting traditional data collection paradigm results in building a series of independent data collection trees when the mobile user is at different positions.