What was the name of the arch on the playa?

[andy]

There was an arch on the playa (maybe Esplanade at 5:00?) where they held a ritual in which people walked under the arch and were sprayed with water, with 4 or 6 knife-wielding guardians (?) lining the entrance. Can anyone tell me what this was called and if this was an ancient ritual or something created for BM? Thanks.