Underground space is an important topic in rock mechanics and rock engineering. The utilization of the characteristics of underground space is considered, and applications to environmental preservation, the effective use of space in mega-cities with high-population densities, the protection of people's lives and property from disasters, etc., are among the topics currently being discussed.

Unfortunately, people generally have negative impressions when they think of being underground, namely, darkness, isolation, fear, etc. In order to make people feel comfortable and pleasant in underground spaces, the psychological viewpoint, in addition to the conventional engineering viewpoint, must be quite important when designing underground spaces. The authors have, therefore, investigated the relationship between mechanics and human impressions for the shapes of underground spaces.

In this paper, the psychological viewpoint is extended to a discussion on "time". Namely, the authors address the question, "How do people feel the progress of time in an underground space?" The time interval which people feel may differ from the interval in clock time. This issue is called "time estimation". Underground spaces should be designed by taking into account the results of this time estimation. In order to investigate the time estimation for underground spaces, a psychological experiment is conducted by using video images. A questionnaire is also used to assess how people feel about the images. These video images were filmed while walking through an underground mall and along outside streets. The results of the experiments are analyzed, and the time estimation, the evaluation of human impressions and the relations among them are discussed.

As a rock engineering project, an investigation into the psychological effects of the time estimation on people will be useful for designing and utilizing underground spaces.

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