Existing audio-visual event localization (AVE) handles manually trimmed videos with only a single instance in each of them. However, this setting is unrealistic as natural videos often contain numerous audio-visual events with different categories. To better adapt to real-life applications, we focus on the task of dense-localizing audio-visual events, which aims to jointly localize and recognize all audio-visual events occurring in an untrimmed video. To tackle this problem, we introduce the first Untrimmed Audio-Visual (UnAV-100) dataset, which contains 10K untrimmed videos with over 30K audio-visual events covering 100 event categories. Each video has 2.8 audio-visual events on average, and the events are usually related to each other and might co-occur as in real-life scenes. We believe our UnAV-100, with its realistic complexity, can promote the exploration on comprehensive audio-visual video understanding.