AbstractFor stormwater systems, geysers in manholes are a risk to pedestrian and vehicle safety and the urban environment. Manhole covers dancing and blowing off are reported worldwide during heavy rainfall and geyser events. These manhole cover motions impact geyser characteristics but are rarely studied. This work experimentally studied geysers in covered manholes induced by air pocket release. Both fixed and mobile covers were tested, and the effects of manhole diameter and cover ventilation size were considered. The geyser regimes are classified into nongeyser, single-geyser, and multigeyser. The mobile covers can dance and blow off, resulting in pressure at the manhole headspace with characteristics of oscillation with high frequency, decline of peak magnitude, and negative pressure occurrence. The maximum positive pressure can be seven times higher for fixed covers than that for mobile covers. Increasing the manhole diameter decreases the number of geysers and the maximum positive pressure for both fixed and mobile covers. Decreasing cover ventilation reduces the number of geysers and increases the maximum positive pressure for the fixed covers, although it has limited effects for mobile covers.