Pat Riley -- Sued Over Leaky Basement

He's crushed the Celtics ... the Bulls ... and now NBA legend Pat Riley is accused of destroying his neighbor's basement ... this according to a new lawsuit obtained by TMZ.

According to the docs, Riley and his wife Christine own a beachfront property in Malibu ... which he overhauled in 2009 with new landscaping, remodeling, terracing and a new irrigation system.

Everything seemed hunky dory ... until Pat's next-door neighbors began to experience some major leakage at their property a short time after the Rileys completed their work.

In the docs, the neighbors allege their special "French drainage" system was collateral damage in the Riley renovation project ... and as a result, their basement flooded. .