def get_experiment_topic(filename):
    """
    Get the experiment topic from the filename.

    The filename is assumed to be in the form of:
    '<reliability>_<durability>_<history kind>_<topic>_<timestamp>'

    :param filename: The filename to get the topic.
    :return: The experiment topic as a string.
    """
    filename = filename.split('/')[-1]
    topic = filename.split('_')[-3]
    return topic